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, 2018	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 668,440	$ 623,103
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	(15,125)	15,495
Acquisitions	20,300	35,666
Sales	0	0
Amortization	2,397	2,489
Other	(9,853)	(10,069)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 666,159	$ 666,684
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	$ 14,049	$ 0
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	(697)	261
Acquisitions	0	14,000
Sales	0	0
Amortization	0	0
Other	(199)	(108)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 13,153	$ 14,153
Percent of total fixed maturities	0.10%	0.10%
Collateralized debt obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 71,581	$ 63,503
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	5,183	3,597
Acquisitions	0	0
Sales	0	0
Amortization	2,389	2,481
Other	(2,957)	(3,336)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 76,196	$ 66,245
Percent of total fixed maturities	0.50%	0.40%
Corporates [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 582,810	$ 559,600
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	(19,611)	11,637
Acquisitions	20,300	21,666
Sales	0	0
Amortization	8	8
Other	(6,697)	(6,625)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 576,810	$ 586,286
Percent of total fixed maturities	3.50%	3.60%