Investment Operations Components of 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 Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									$ 432,530	$ 403,065	$ 422,666
Derivative income (loss)									25,282	(10,405)	7,687
Prepayment fee income and other									7,332	9,208	12,470
Less investment expenses									(7,532)	(8,447)	(7,467)
Net investment income	$ 108,986	$ 101,478	$ 104,894	$ 109,640	$ 83,865	$ 105,757	$ 103,974	$ 101,022	424,998	394,618	415,199
Fixed maturities - available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									334,557	340,498	344,302
Equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									8,718	8,488	6,502
Mortgage loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									46,182	45,294	42,185
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									9,416	9,210	9,014
Short-term investments, cash and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									$ 1,043	$ 772	$ 506