Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Level 3 Inputs) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of the period		$ (2,680)		$ 1,719
Total realized and unrealized gains
Purchases		(10,127)		(10,700)
Settlements				(7,500)
Balance at end of the period		(12,012)		(12,012)
Net investment income
Total realized and unrealized gains
Total realized and unrealized gains (losses)				(118)
Other income (loss)
Total realized and unrealized gains
Total realized and unrealized gains (losses)		795		4,587
Other assets and (liabilities)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of the period	$ (9,502)	(2,680)	$ (13,004)	(5,899)
Total realized and unrealized gains
Purchases	(63)	(10,127)	49	(10,700)
Settlements	5,937		5,937	0
Balance at end of the period	(3,493)	(12,012)	(3,493)	(12,012)
Other assets and (liabilities) | Net investment income
Total realized and unrealized gains
Total realized and unrealized gains (losses)				0
Other assets and (liabilities) | Other income (loss)
Total realized and unrealized gains
Total realized and unrealized gains (losses)	$ 135	795	$ 3,525	4,587
Fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of the period		0		7,618
Total realized and unrealized gains
Purchases		0		0
Settlements				(7,500)
Balance at end of the period		0		0
Fixed maturity investments | Net investment income
Total realized and unrealized gains
Total realized and unrealized gains (losses)				(118)
Fixed maturity investments | Other income (loss)
Total realized and unrealized gains
Total realized and unrealized gains (losses)		$ 0		$ 0