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, 2018	Sep. 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	698	0
Included in other comprehensive income	(14,113)	18,996
Acquisitions	27,453	35,666
Sales	0	0
Amortization	3,585	3,719
Other	(42,862)	(12,850)
Transfers in and/or out of Level 3	4,533	0
Ending balance	$ 647,734	$ 668,634
Percent of total fixed maturities	4.00%	4.00%
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	(950)	595
Acquisitions	0	14,000
Sales	0	0
Amortization	0	0
Other	(315)	(212)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 12,784	$ 14,383
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	8,691	7,787
Acquisitions	0	0
Sales	0	0
Amortization	3,572	3,705
Other	(4,928)	(5,227)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 78,916	$ 69,768
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	698	0
Included in other comprehensive income	(21,854)	10,614
Acquisitions	27,453	21,666
Sales	0	0
Amortization	13	14
Other	(37,619)	(7,411)
Transfers in and/or out of Level 3	4,533	0
Ending balance	$ 556,034	$ 584,483
Percent of total fixed maturities	3.40%	3.50%