Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive (loss) income	$ (14,730)	$ 17,535	$ (107,759)	$ 75,387
Income tax benefit (expense)	3,093	(6,138)	22,629	(26,384)
Other comprehensive (loss) income, net of tax	(11,637)	11,397	(85,130)	49,003
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Net realized investment gains (losses)	1,114	(50)	(1,112)	(227)
Other underwriting and operating expenses, net	(43,655)	(39,888)	(131,587)	(119,164)
Income before tax	231,894	184,467	649,441	539,040
Income tax benefit (expense)	(49,994)	(64,440)	(137,090)	(210,593)
Net income	181,900	120,027	512,351	328,447
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			3,154,526
Other comprehensive income before reclassifications			(91,413)
Less: Amounts reclassified from AOCL			(6,283)
Balance, end of period	3,489,531	3,130,149	3,489,531	3,130,149
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	(3,125)	(201)	(7,953)	(1,886)
Income tax benefit (expense)	657	71	1,670	660
Net income	(2,468)	(130)	(6,283)	(1,226)
Accumulated other comprehensive (loss) income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive (loss) income, net of tax			(85,130)	49,003
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(43,801)	(75,100)
Balance, end of period	(128,931)	(26,097)	(128,931)	(26,097)
Net unrealized gains and losses on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive (loss) income	(15,288)	17,761	(109,433)	76,022
Income tax benefit (expense)	3,211	(6,217)	22,981	(26,608)
Other comprehensive (loss) income, net of tax	(12,077)	11,544	(86,452)	49,414
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			29,257
Other comprehensive income before reclassifications			(91,413)
Less: Amounts reclassified from AOCL			(4,961)
Balance, end of period	(57,195)		(57,195)
Net unrealized gains and losses on available-for-sale securities | Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Net realized investment gains (losses)	(2,567)	(427)	(6,279)	(2,566)
Income tax benefit (expense)	539	150	1,318	898
Net income	(2,028)	(277)	(4,961)	(1,668)
Net benefit plan assets and obligations recognized in shareholders' equity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive (loss) income	558	(226)	1,674	(680)
Income tax benefit (expense)	(118)	79	(352)	238
Other comprehensive (loss) income, net of tax	440	(147)	1,322	(442)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(73,058)
Other comprehensive income before reclassifications			0
Less: Amounts reclassified from AOCL			(1,322)
Balance, end of period	(71,736)		(71,736)
Net benefit plan assets and obligations recognized in shareholders' equity | Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Other underwriting and operating expenses, net	(558)	226	(1,674)	680
Income tax benefit (expense)	118	(79)	352	(238)
Net income	(440)	147	(1,322)	442
Net unrealized foreign currency translation
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive (loss) income	0	0	0	45
Income tax benefit (expense)	0	0	0	(14)
Other comprehensive (loss) income, net of tax	$ 0	$ 0	$ 0	$ 31