Investments - Schedule of Changes in Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 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	788
Included in other comprehensive income	18,996	29,486
Acquisitions	35,666	33,662
Sales	0	0
Amortization	3,719	3,525
Other	(12,850)	(17,614)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 668,634	$ 651,035
Percent of total fixed maturities	4.00%	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	595	0
Acquisitions	14,000	0
Sales	0	0
Amortization	0	0
Other	(212)	0
Transfers in and/or out of Level 3	0	0
Ending balance	$ 14,383	$ 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	7,787	(3,879)
Acquisitions	0	0
Sales	0	0
Amortization	3,705	3,511
Other	(5,227)	(6,732)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 69,768	$ 63,282
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	788
Included in other comprehensive income	10,614	33,365
Acquisitions	21,666	33,662
Sales	0	0
Amortization	14	14
Other	(7,411)	(10,882)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 584,483	$ 587,753
Percent of total fixed maturities	3.50%	3.70%