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, 2018	Mar. 31, 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	(8,361)	11,365
Acquisitions	0	35,666
Sales	0	0
Amortization	1,205	1,253
Other	(3,773)	(3,556)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 657,511	$ 667,831
Percent of total fixed maturities	4.00%	4.20%
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	(749)	0
Acquisitions	0	14,000
Sales	0	0
Amortization	0	0
Other	(113)	0
Transfers in and/or out of Level 3	0	0
Ending balance	$ 13,187	$ 14,000
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	4,621	5,090
Acquisitions	0	0
Sales	0	0
Amortization	1,201	1,249
Other	(1,879)	(1,999)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 75,524	$ 67,843
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	(12,233)	6,275
Acquisitions	0	21,666
Sales	0	0
Amortization	4	4
Other	(1,781)	(1,557)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 568,800	$ 585,988
Percent of total fixed maturities	3.40%	3.70%