Consolidated Statement of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period	$ 55,568
Other comprehensive gain (loss)	12,954	$ 122,325	$ (250,286)
Ending balance	68,541	55,568
Income before income taxes	772,770	896,438	732,030
Other operating costs and expenses	2,436,932	2,395,619	2,289,750
Federal and foreign income taxes	(219,433)	(292,953)	(227,923)
Net income before noncontrolling interests	553,337	603,485	504,107
Unrealized investment gains, before tax	(69,425)	379,258
Currency translation adjustments, before tax	64,706	(124,193)
Other comprehensive income (loss), before tax	(4,719)	255,065
Unrealized investment gains, tax	17,673	(132,740)
Currency translation adjustments, tax	0	0
Other comprehensive income (loss), tax	17,673	(132,740)
Unrealized investment gains, net of tax	(51,752)	246,518
Change in unrealized translation adjustments	64,706	(124,193)	(124,744)
Unrealized investment gains (losses)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period	427,154	180,695
Other comprehensive income before reclassifications	63,567	286,734
Amounts reclassified from AOCI	(115,319)	(40,216)
Other comprehensive gain (loss)	(51,752)	246,518
Unrealized investment gain related to non-controlling interest	19	(59)
Ending balance	375,421	427,154	180,695
Unrealized investment gains (losses) | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net investment gains	(177,414)	(61,871)
Federal and foreign income taxes	62,095	21,655
Net income before noncontrolling interests	(115,319)	(40,216)
Currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period	(371,586)	(247,393)
Other comprehensive income before reclassifications	64,706	(124,193)
Amounts reclassified from AOCI	0	0
Other comprehensive gain (loss)	64,706	(124,193)
Unrealized investment gain related to non-controlling interest	0	0
Ending balance	(306,880)	(371,586)	(247,393)
Change in unrealized translation adjustments	64,706	(124,193)	(124,744)
Currency translation adjustments | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	0	0
Federal and foreign income taxes		0
Net income before noncontrolling interests	0	0
Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period	55,568	(66,698)
Other comprehensive income before reclassifications	128,273	162,541
Amounts reclassified from AOCI	(115,319)	(40,216)
Other comprehensive gain (loss)	12,954	122,325
Unrealized investment gain related to non-controlling interest	19	(59)
Ending balance	68,541	55,568	$ (66,698)
Accumulated other comprehensive income (loss) | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	(177,414)	(61,871)
Federal and foreign income taxes	62,095	21,655
Net income before noncontrolling interests	(115,319)	(40,216)
Unrealized investment gains (losses)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning of period	$ 427,154
Ending balance		$ 427,154