Investments - Schedule of Changes in Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 623,103	$ 601,188
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	11,365	5,970
Acquisitions	35,666	15,800
Sales	0	0
Amortization	1,253	1,338
Other	(3,556)	(4,003)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 667,831	$ 620,293
Percent of total fixed maturities	4.20%	4.30%
Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 0	$ 0
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	0	0
Acquisitions	14,000	0
Sales	0	0
Amortization	0	0
Other	0	0
Transfers in and/or out of Level 3	0	0
Ending balance	$ 14,000	$ 0
Percent of total fixed maturities	0.10%	0.00%
Collateralized debt obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 63,503	$ 70,382
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	5,090	(3,598)
Acquisitions	0	0
Sales	0	0
Amortization	1,249	1,334
Other	(1,999)	(2,626)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 67,843	$ 65,492
Percent of total fixed maturities	0.40%	0.50%
Corporates [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 559,600	$ 530,806
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	6,275	9,568
Acquisitions	21,666	15,800
Sales	0	0
Amortization	4	4
Other	(1,557)	(1,377)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 585,988	$ 554,801
Percent of total fixed maturities	3.70%	3.80%