Fair Value - Valuation Technique and Inputs (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair Value Inputs Assets Quantitative Information [Line Items]
Fixed maturities	13,918,745	13,593,303
Equities	1,056,514	1,221,053
Other invested assets carried at fair value	111,476	135,139
Funds held directly managed investments carried at fair value	472,593	561,306
Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fixed maturities	599,646	554,957
Equities	40,134	38,045
Other invested assets carried at fair value	116,074	103,853
Funds held directly managed investments carried at fair value	13,530	15,451
U.S. states, territories and municipalities | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fixed maturities	149,728	108,380
U.S. states, territories and municipalities | Minimum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	2.20%	2.90%
U.S. states, territories and municipalities | Maximum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	10.10%	9.90%
U.S. states, territories and municipalities | Weighted Average [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	4.60%	5.30%
Asset-backed securities - interest only [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fixed maturities		21
Asset-backed securities - interest only [Member] | Minimum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads		5.50%
Asset-backed securities - interest only [Member] | Maximum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads		10.70%
Asset-backed securities - interest only [Member] | Weighted Average [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads		8.80%
Asset-backed securities - other [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Fixed maturities	449,918	446,556
Asset-backed securities - other [Member] | Minimum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	4.00%	4.00%
Asset-backed securities - other [Member] | Maximum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	12.10%	12.20%
Asset-backed securities - other [Member] | Weighted Average [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	7.10%	7.10%
Finance [Member] | Weighted market comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Equities	14,561	15,483
Finance [Member] | Profitability Analysis [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Equities	5,792	4,724
Finance [Member] | Minimum [Member] | Weighted market comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net income multiple	19.0	14.6
Tangible book value multiple	1.3	1.1
Liquidity discount	25.00%	25.00%
Comparable return	7.30%	8.50%
Finance [Member] | Minimum [Member] | Profitability Analysis [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Projected Return On Equity	14.00%	14.00%
Finance [Member] | Maximum [Member] | Weighted market comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net income multiple	19.0	14.6
Tangible book value multiple	1.3	1.1
Liquidity discount	25.00%	25.00%
Comparable return	7.30%	8.50%
Finance [Member] | Maximum [Member] | Profitability Analysis [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Projected Return On Equity	14.00%	14.00%
Finance [Member] | Weighted Average [Member] | Weighted market comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net income multiple	19.0	14.6
Tangible book value multiple	1.3	1.1
Liquidity discount	25.00%	25.00%
Comparable return	7.30%	8.50%
Finance [Member] | Weighted Average [Member] | Profitability Analysis [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Projected Return On Equity	14.00%	14.00%
Technology [Member] | Weighted market comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Equities	8,555	7,752
Technology [Member] | Minimum [Member] | Weighted market comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjusted earnings multiple	10.2	4.4
Gross revenue/fair value	1.6	0.9
Technology [Member] | Maximum [Member] | Weighted market comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjusted earnings multiple	10.2	4.4
Gross revenue/fair value	1.6	0.9
Technology [Member] | Weighted Average [Member] | Weighted market comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjusted earnings multiple	10.2	4.4
Gross revenue/fair value	1.6	0.9
Communications [Member] | Weighted market comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Equities	2,640	2,199
Communications [Member] | Minimum [Member] | Weighted market comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjusted earnings multiple	9.4	9.4
Comparable return	(10.60%)	0.00%
Communications [Member] | Maximum [Member] | Weighted market comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjusted earnings multiple	9.4	9.4
Comparable return	(10.60%)	0.00%
Communications [Member] | Weighted Average [Member] | Weighted market comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Adjusted earnings multiple	9.4	9.4
Comparable return	(10.60%)	0.00%
Total return swaps [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Other invested assets carried at fair value	(1,522)	(520)
Total return swaps [Member] | Minimum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	3.60%	2.80%
Total return swaps [Member] | Maximum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	19.30%	18.90%
Total return swaps [Member] | Weighted Average [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	16.30%	17.00%
Notes and loan receivables [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Other invested assets carried at fair value	13,237	21,280
Notes and loan receivables [Member] | Minimum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	17.50%	17.50%
Gross revenue/fair value	1.5	1.5
Notes and loan receivables [Member] | Maximum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	17.50%	17.50%
Gross revenue/fair value	1.7	1.5
Notes and loan receivables [Member] | Weighted Average [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	17.50%	17.50%
Gross revenue/fair value	1.7	1.5
Notes and Loans Receivable 2 [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Other invested assets carried at fair value	8,068
Notes and Loans Receivable 2 [Member] | Minimum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	12.60%
Notes and Loans Receivable 2 [Member] | Maximum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	12.60%
Notes and Loans Receivable 2 [Member] | Weighted Average [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	12.60%
Notes securitization [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Other invested assets carried at fair value	23,512	20,166
Notes securitization [Member] | Minimum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	3.50%	6.20%
Notes securitization [Member] | Maximum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	6.60%	6.20%
Notes securitization [Member] | Weighted Average [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	6.40%	6.20%
Annuities and residuals [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Other invested assets carried at fair value	13,243	24,064
Annuities and residuals [Member] | Minimum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	4.90%	4.00%
Prepayment speed	0.00%	0.00%
Constant default rate	0.30%	0.30%
Annuities and residuals [Member] | Maximum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	9.60%	7.90%
Prepayment speed	15.00%	15.00%
Constant default rate	17.50%	35.00%
Annuities and residuals [Member] | Weighted Average [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads	7.80%	5.80%
Prepayment speed	4.30%	6.40%
Constant default rate	6.30%	12.40%
Private Equity-Direct [Member] | Discounted Cash Flow And Weighted Market Comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Other invested assets carried at fair value	8,536	11,742
Private Equity-Direct [Member] | Minimum [Member] | Discounted Cash Flow And Weighted Market Comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net income multiple	9.0	8.3
Tangible book value multiple	2.0	1.6
Recoverability of intangible assets	0.000	0.000
Private Equity-Direct [Member] | Maximum [Member] | Discounted Cash Flow And Weighted Market Comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net income multiple	9.0	8.3
Tangible book value multiple	2.0	1.6
Recoverability of intangible assets	0.000	0.000
Private Equity-Direct [Member] | Weighted Average [Member] | Discounted Cash Flow And Weighted Market Comparables [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net income multiple	9.0	8.3
Tangible book value multiple	2.0	1.6
Recoverability of intangible assets	0.000	0.000
Private equity funds [Member] | Reported market value | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Other invested assets carried at fair value	18,494	8,993
Private equity funds [Member] | Minimum [Member] | Reported market value | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net asset value, as reported	100.00%	100.00%
Market adjustments	(7.60%)	1.80%
Private equity funds [Member] | Maximum [Member] | Reported market value | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net asset value, as reported	100.00%	100.00%
Market adjustments	11.00%	9.80%
Private equity funds [Member] | Weighted Average [Member] | Reported market value | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net asset value, as reported	100.00%	100.00%
Market adjustments	(1.60%)	8.30%
Private Equity - Other [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Other invested assets carried at fair value	32,842	18,396
Private Equity - Other [Member] | Minimum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads		3.80%
Effective Yield	5.80%
Private Equity - Other [Member] | Maximum [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads		3.80%
Effective Yield	5.80%
Private Equity - Other [Member] | Weighted Average [Member] | Discounted cash flow [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Credit spreads		3.80%
Effective Yield	5.80%
Other invested assets [Member] | Reported market value | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Funds held directly managed investments carried at fair value	13,398	15,165
Other invested assets [Member] | Minimum [Member] | Reported market value | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net asset value, as reported	100.00%	100.00%
Market adjustments	(15.40%)	(22.90%)
Other invested assets [Member] | Maximum [Member] | Reported market value | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net asset value, as reported	100.00%	100.00%
Market adjustments	0.00%	0.00%
Other invested assets [Member] | Weighted Average [Member] | Reported market value | Significant unobservable inputs (Level 3) [Member]
Fair Value Inputs Assets Quantitative Information [Line Items]
Net asset value, as reported	100.00%	100.00%
Market adjustments	(14.50%)	(15.50%)