Fair Value Measurements (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	$ (17,816,000)		$ (17,816,000)		$ (9,362,000)
Debt	951,620,000		951,620,000		960,495,000
Debt, fair value	1,000,000,000		1,000,000,000		$ 973,300,000
Net unrealized gains (losses) recognized in earnings			(17,608,000)	$ (10,690,000)
Other Investments | Net investment income
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net unrealized gains (losses) recognized in earnings	2,800,000	$ (13,900,000)	17,600,000	(10,700,000)
Other assets and (liabilities) | Other income (loss)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net unrealized gains (losses) recognized in earnings	0	$ (100,000)	0	$ (400,000)
Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	12,012,000		12,012,000
Significant Unobservable Inputs (Level 3) | Assumed reinsurance contract | Internal Valuation Model Valuation Technique
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	(677,000)		$ (677,000)
Significant Unobservable Inputs (Level 3) | Assumed reinsurance contract | Weighted Average or Actual | Internal Valuation Model Valuation Technique
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidity discount			1.30%
Significant Unobservable Inputs (Level 3) | Assumed and ceded (re)insurance contract | Internal Valuation Model Valuation Technique
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	$ (11,335,000)		$ (11,335,000)
U.S. treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	0.90%		0.90%		1.30%
Agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.30%		1.30%		1.70%
Municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.70%		1.70%		2.00%
Non-U.S. government (Sovereign Debt)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.10%		1.10%		1.40%
Non-U.S. government-backed corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.00%		1.00%		1.30%
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	3.50%		3.50%		3.80%
Agency mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	2.10%		2.10%		2.70%
Weighted average life			5 years		6 years 1 month 6 days
Non-agency prime residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	4.20%		4.20%		3.80%
Weighted average life			4 years 6 months		4 years 3 months 18 days
AltA non-agency mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	5.00%		5.00%		4.70%
Weighted average life			5 years 10 months 24 days		5 years 4 months 24 days
Commercial mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	2.30%		2.30%		2.90%
Weighted average life			3 years 8 months 12 days		3 years 8 months 12 days
Asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	2.10%		2.10%		2.10%
Weighted average life			2 years 9 months 18 days		2 years 6 months
Short term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	0.50%		0.50%		0.40%
Restricted stock units | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	$ (8,200,000)		$ (8,200,000)
Private equity partnerships | Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets			7 years
Private equity partnerships | Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets			10 years
Senior secured bank loan funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment in closed end fund	19,400,000		$ 19,400,000
Senior secured bank loan funds | Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets			4 years
Senior secured bank loan funds | Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets			5 years
Hedge funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Hedge fund side pocket investments	$ 1,600,000		$ 1,600,000