Fair Value of Financial Instruments - Rollforward of Level 3 Assets Measured on Recurring Basis (Details) - Fair Value, Recurring Measurements [Member] - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Level 3 assets, beginning of period		$ 8,162	$ 783
Net unrealized gains (losses) included in other comprehensive income		232	(23)
Realized gains (losses) and accretion (amortization) recognized in earnings, including OTTI		26	7
Purchases			7,556
Settlements		(958)	(162)
Transfers into Level 3			1
Transfers out of Level 3	[1]	(6,895)
Level 3 assets, end of period		$ 567	$ 8,162

[1] During the year ended December 31, 2017, transfers out of Level 3 assets primarily consisted of newly issued fixed-maturity securities purchased in the fourth quarter of 2016 for which observable inputs, most notably quoted prices, used to derive valuations were not yet readily available.