Investments - Net Investment Income (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating										$ 151,254	$ 127,031	$ 111,343
Investment expenses										(8,856)	(8,116)	(5,461)
Investment income net of investment expenses										142,398	118,915	105,882
Interest expense on surplus note										(48,325)	(37,485)	(26,865)
Net investment income		$ 22,418	$ 22,675	$ 24,868	$ 24,111	$ 21,760	$ 20,622	$ 20,030	$ 19,017	94,073	81,430	79,017
Equity Securities [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating										1,845	1,955	2,095
Policy Loans and Other Invested Assets [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating										1,069	1,159	1,179
Cash and Cash Equivalents [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating										4,758	3,433	1,357
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]									13,429	3,643	2,970
Available-for-Sale [Member] | Fixed-maturity Securities [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating										81,828	79,356	76,877
Held-to-Maturity [Member] | Fixed-maturity Securities [Member]
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income, operating										$ 48,325	$ 37,485	$ 26,865

[1] ⁽¹⁾ Includes $5.4 million of net gains and $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, 2019 and December 31, 2018, respectively. The change in fair value of the deposit asset underlying the 10% coinsurance agreement for the year ended December 31, 2017 was not material.