Fair Value Measurements (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	$ 3,660,000		$ 3,660,000		$ 9,362,000
Debt	954,577,000		954,577,000		960,495,000
Debt, fair value	1,008,500,000		1,008,500,000		$ 973,300,000
Net unrealized gains (losses) recognized in earnings			(20,360,000)	$ 3,238,000
Other Investments | Net investment income
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net unrealized gains (losses) recognized in earnings	(5,000,000)	$ (1,600,000)	(20,400,000)	3,200,000
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	$ (300,000)	0	$ (300,000)
Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	2,680,000		2,680,000
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	(1,302,000)		$ (1,302,000)
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	$ (1,378,000)		$ (1,378,000)
U.S. treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	0.70%		0.70%		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	1.60%		1.60%		2.00%
Non-U.S. government (Sovereign Debt)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.00%		1.00%		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	3.70%		3.70%		3.80%
Agency mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	2.00%		2.00%		2.70%
Weighted average life			4 years 9 months 18 days		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	4.40%		4.40%		3.80%
Weighted average life			4 years 2 months 12 days		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	5.50%		5.50%		4.70%
Weighted average life			5 years 6 months		5 years 4 months 24 days
Commercial mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	2.80%		2.80%		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	1.90%		1.90%		2.10%
Weighted average life			2 years 6 months		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	$ 4,500,000		$ 4,500,000
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	22,200,000		$ 22,200,000
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