Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive (loss) income	$ (12,067)	$ 39,394	$ (93,029)	$ 57,852
Income tax benefit (expense)	2,533	(13,787)	19,536	(20,246)
Other comprehensive (loss) income, net of tax	(9,534)	25,607	(73,493)	37,606
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Net realized investment losses	(1,897)	(52)	(2,226)	(177)
Other underwriting and operating expenses, net	(41,842)	(38,511)	(87,932)	(79,276)
Income before tax	237,522	180,616	417,547	354,573
Income tax benefit (expense)	(50,708)	(61,994)	(87,096)	(146,153)
Net income	186,814	118,622	330,451	208,420
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			3,154,526
Other comprehensive income before reclassifications			(77,308)
Less: Amounts reclassified from AOCL			(3,815)
Balance, end of period	3,313,875	2,995,061	3,313,875	2,995,061
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	(4,112)	(1,172)	(4,828)	(1,685)
Income tax benefit (expense)	863	409	1,013	589
Net income	(3,249)	(763)	(3,815)	(1,096)
Accumulated other comprehensive (loss) income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive (loss) income, net of tax			(73,493)	37,606
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(43,801)	(75,100)
Balance, end of period	(117,294)	(37,494)	(117,294)	(37,494)
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	(12,558)	39,614	(94,145)	58,261
Income tax benefit (expense)	2,636	(13,865)	19,770	(20,391)
Other comprehensive (loss) income, net of tax	(9,922)	25,749	(74,375)	37,870
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			29,257
Other comprehensive income before reclassifications			(77,308)
Less: Amounts reclassified from AOCL			(2,933)
Balance, end of period	(45,118)		(45,118)
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 losses	(3,621)	(1,392)	(3,712)	(2,139)
Income tax benefit (expense)	760	487	779	748
Net income	(2,861)	(905)	(2,933)	(1,391)
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	491	(220)	1,116	(454)
Income tax benefit (expense)	(103)	78	(234)	159
Other comprehensive (loss) income, net of tax	388	(142)	882	(295)
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			(882)
Balance, end of period	(72,176)		(72,176)
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	(491)	220	(1,116)	454
Income tax benefit (expense)	103	(78)	234	(159)
Net income	(388)	142	(882)	295
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