Investment Operations Components of Net Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									$ 411,561	$ 399,436	$ 390,121
Gain (Loss) on Derivative Instruments, Net, Pretax									3,935	(2,266)	2,496
Prepayment fee income and other									10,992	11,555	6,959
Less investment expenses									(7,391)	(8,287)	(8,039)
Net investment income	$ 101,549	$ 103,514	$ 100,722	$ 98,385	$ 99,005	$ 95,882	$ 97,489	$ 98,773	404,170	391,149	382,082
Fixed Maturities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									342,657	338,952	333,759
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									6,558	6,091	5,388
Mortgages Loans [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									38,098	35,923	32,759
Real Estate [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									0	169	140
Policy Loans [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									8,956	8,871	8,620
Short-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross Investment Income, Operating									$ 365	$ 141	$ 0