Investments - Schedule of Changes in Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 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	15,495	19,460
Acquisitions	35,666	15,800
Sales	0	0
Amortization	2,489	2,647
Other	(10,069)	(5,867)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 666,684	$ 633,228
Percent of total fixed maturities	4.10%	4.10%
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	261	0
Acquisitions	14,000	0
Sales	0	0
Amortization	0	0
Other	(108)	0
Transfers in and/or out of Level 3	0	0
Ending balance	$ 14,153	$ 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	3,597	(4,831)
Acquisitions	0	0
Sales	0	0
Amortization	2,481	2,639
Other	(3,336)	(4,127)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 66,245	$ 64,063
Percent of total fixed maturities	0.40%	0.40%
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	11,637	24,291
Acquisitions	21,666	15,800
Sales	0	0
Amortization	8	8
Other	(6,625)	(1,740)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 586,286	$ 569,165
Percent of total fixed maturities	3.60%	3.70%