Investments - Net Investment Income (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating										$ 127,031	$ 111,343	$ 102,790
Investment expenses										(8,116)	(5,461)	(4,885)
Investment income net of investment expenses										118,915	105,882	97,905
Interest expense on surplus note										(37,485)	(26,865)	(18,880)
Net investment income		$ 21,760	$ 20,622	$ 20,030	$ 19,017	$ 19,459	$ 19,922	$ 19,742	$ 19,894	81,430	79,017	79,025
Equity Securities [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating										1,955	2,095	2,053
Policy Loans and Other Invested Assets [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating										1,670	1,413	1,340
Cash and Cash Equivalents [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating										2,922	1,123	632
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	[1]									3,643	2,970	5,212
Available-for-Sale [Member] | Fixed-maturity Securities [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating										79,356	76,877	74,673
Held-to-Maturity [Member] | Fixed-maturity Securities [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating										$ 37,485	$ 26,865	$ 18,880

[1] ^{Includes $1.7 million of net losses recognized for the change in fair value of the deposit asset underlying the 10% coinsurance agreement for the year ended December 31, 2018. The change in fair value of the deposit asset underlying the 10% coinsurance agreement for the years ended December 31, 2017 and 2016 was not material.}