Investments - Net Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating									$ 111,343	$ 102,790	$ 94,456
Investment expenses									(5,461)	(4,885)	(4,899)
Investment income net of investment expenses									105,882	97,905	89,557
Interest expense on surplus note									(26,865)	(18,880)	(13,048)
Net investment income	$ 19,459	$ 19,922	$ 19,742	$ 19,894	$ 17,999	$ 19,399	$ 20,389	$ 21,238	79,017	79,025	76,509
Equity Securities [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating									2,095	2,053	2,059
Policy Loans and Other Invested Assets [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating									1,413	1,340	1,368
Cash and Cash Equivalents [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating									1,123	632	228
Total return on deposit asset underlying 10% insurance agreement [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating									2,970	5,212	482
Available-for-Sale [Member] | Fixed-maturity Securities [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating									76,877	74,673	77,271
Held-to-Maturity [Member] | Fixed-maturity Securities [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating									$ 26,865	$ 18,880	$ 13,048