Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	$ 5,876		$ 5,876		$ 17,383
Debt	989,245		989,245		948,663
Debt, Fair value	1,034,200		1,034,200		964,800
Net unrealized gains (losses) recognized in earnings			2,686	$ (17,608)
Net investment income | Other Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net unrealized gains (losses) recognized in earnings	(9,800)	$ 9,400	2,700	$ 6,100
Other income (loss) | Other assets and (liabilities)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net unrealized gains (losses) recognized in earnings	0	$ 0	0		$ 0
Significant Unobservable Inputs (Level 3) | Internal Valuation Model
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	3,493		3,493
Significant Unobservable Inputs (Level 3) | Assumed reinsurance contract | Internal Valuation Model
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	582		582
Significant Unobservable Inputs (Level 3) | Assumed and ceded (re)insurance contract | Internal Valuation Model
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	$ 4,075		$ 4,075
U.S. treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.60%		1.60%		1.40%
Agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.60%		1.60%		2.00%
Municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.90%		1.90%		2.40%
Non-U.S. government (Sovereign debt)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.70%		1.70%		1.60%
Non-U.S. government-backed corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.90%		1.90%		1.50%
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	3.50%		3.50%		3.70%
Agency mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	2.90%		2.90%		2.90%
Weighted average life			6 years 3 months 18 days		6 years 10 months 24 days
Non-agency prime residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	3.80%		3.80%		4.30%
Weighted average life			5 years		5 years 1 month 6 days
AltA non-agency mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	3.70%		3.70%		5.20%
Weighted average life			6 years 2 months 12 days		6 years
Commercial mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	3.10%		3.10%		2.60%
Weighted average life			4 years 6 months		3 years 10 months 24 days
Asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	2.40%		2.40%		2.30%
Weighted average life			2 years 9 months 18 days		2 years 7 months 6 days
Short term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	1.10%		1.10%		0.70%
Restricted stock units | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	$ 8,800		$ 8,800
Private equity partnerships | Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets			7 years
Private equity partnerships | High
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	19,600		$ 19,600
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 | High
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	$ 600		$ 600