CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 82,542	$ 93,294	$ 242,224	$ 320,762
Other comprehensive income (loss):
Change in net unrealized gains (losses) on investments, net of income tax: (three months: 2017 - $54,464; 2016 - $106,841; nine months: 2017 - $295,743; 2016 - $657,352)	101,148	198,419	549,234	1,220,796
Reclassification adjustment for investment amounts included in net income, net of income tax: (three months: 2017 - $(146); 2016 - $575; nine months: 2017 - $631; 2016 - $(6,041))	(271)	1,068	1,174	(11,219)
Change in net unrealized gains (losses) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings, net of income tax: (three months: 2017 - $452; 2016 - $1,278; nine months: 2017 - $3,837; 2016 - $(106))	839	2,374	7,125	(198)
Change in accumulated (loss) gain - derivatives, net of income tax: (three months: 2017 - $445; 2016 - $0; nine months: 2017 - $19; 2016 - $0)	828	0	36	0
Reclassification adjustment for derivative amounts included in net income, net of income tax: (three months: 2017 - $14; 2016 - $0; nine months: 2017 - $139; 2016 - $0)	24	0	257	0
Change in postretirement benefits liability adjustment, net of income tax: (three months: 2017 - $0; 2016 - $(874); nine months: 2017 - $0; 2016 - $(874))	0	(1,624)	0	(1,624)
Total other comprehensive income	102,568	200,237	557,826	1,207,755
Total comprehensive income	$ 185,110	$ 293,531	$ 800,050	$ 1,528,517