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, 2019	Mar. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 639,822	$ 668,440
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	15,137	(8,361)
Acquisitions	0	0
Sales	0	0
Amortization	1,165	1,205
Other	(3,401)	(3,773)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 652,723	$ 657,511
Percent of total fixed maturities	3.80%	4.00%
Asset-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 12,982	$ 14,049
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	298	(749)
Acquisitions	0	0
Sales	0	0
Amortization	0	0
Other	(126)	(113)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 13,154	$ 13,187
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	$ 73,369	$ 71,581
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	4,176	4,621
Acquisitions	0	0
Sales	0	0
Amortization	1,162	1,201
Other	(1,458)	(1,879)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 77,249	$ 75,524
Percent of total fixed maturities	0.40%	0.50%
Corporates [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 553,471	$ 582,810
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	10,663	(12,233)
Acquisitions	0	0
Sales	0	0
Amortization	3	4
Other	(1,817)	(1,781)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 562,320	$ 568,800
Percent of total fixed maturities	3.30%	3.40%