Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive (loss) income	$ (80,962)	$ 18,458
Income tax benefit (expense)	17,003	(6,459)
Other comprehensive (loss) income, net of tax	(63,959)	11,999
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Net realized investment losses	(329)	(125)
Other underwriting and operating expenses, net	(46,090)	(40,765)
Income before tax	180,025	173,957
Income tax benefit (expense)	(36,388)	(84,159)
Net income	143,637	89,798
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	3,154,526
Other comprehensive income before reclassifications	(64,525)
Less: Amounts reclassified from AOCL	(566)
Balance, end of period	3,231,403	2,647,531
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	(716)	(513)
Income tax benefit (expense)	150	180
Net income	(566)	(333)
Accumulated other comprehensive (loss) income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive (loss) income, net of tax	(63,959)	11,999
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(43,801)	(75,100)
Balance, end of period	(107,760)	(63,101)
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	(81,587)	18,647
Income tax benefit (expense)	17,134	(6,526)
Other comprehensive (loss) income, net of tax	(64,453)	12,121
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	29,257
Other comprehensive income before reclassifications	(64,525)
Less: Amounts reclassified from AOCL	(72)
Balance, end of period	(35,196)
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	(91)	(747)
Income tax benefit (expense)	19	261
Net income	(72)	(486)
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	625	(234)
Income tax benefit (expense)	(131)	81
Other comprehensive (loss) income, net of tax	494	(153)
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	(494)
Balance, end of period	(72,564)
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	(625)	234
Income tax benefit (expense)	131	(81)
Net income	(494)	153
Net unrealized foreign currency translation
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive (loss) income	0	45
Income tax benefit (expense)	0	(14)
Other comprehensive (loss) income, net of tax	$ 0	$ 31