Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	$ 18,458	$ 76,413
Income tax (expense) benefit	(6,459)	(26,869)
Other comprehensive income, net of tax	11,999	49,544
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Net realized investment (losses) gains	(122)	3,056
Other underwriting and operating expenses, net	(40,765)	(39,777)
Other revenue	2,422	6,373
Reclassification adjustment	173,957	103,688
Income tax benefit (expense)	(84,159)	(34,497)
Net income	89,798	69,191
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	2,548,842
Other comprehensive income before reclassifications	11,666
Less: Amounts reclassified from AOCL	(333)
Balance, end of period	2,647,531	2,346,812
Accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive income, net of tax	11,999	49,544
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(75,100)	(60,880)
Balance, end of period	(63,101)	(11,336)
Net unrealized gains and losses on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	18,647	78,383
Income tax (expense) benefit	(6,526)	(27,556)
Other comprehensive income, net of tax	12,121	50,827
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(20,797)
Other comprehensive income before reclassifications	11,635
Less: Amounts reclassified from AOCL	(486)
Balance, end of period	(8,676)
Net benefit plan assets and obligations recognized in shareholders' equity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	(234)	(474)
Income tax (expense) benefit	81	166
Other comprehensive income, net of tax	(153)	(308)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(54,272)
Other comprehensive income before reclassifications	0
Less: Amounts reclassified from AOCL	153
Balance, end of period	(54,425)
Net unrealized foreign currency translation
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	45	(1,496)
Income tax (expense) benefit	(14)	521
Other comprehensive income, net of tax	31	(975)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(31)
Other comprehensive income before reclassifications	31
Less: Amounts reclassified from AOCL	0
Balance, end of period	0
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Reclassification adjustment	(513)	(381)
Income tax benefit (expense)	180	255
Net income	(333)	(126)
Reclassification from Accumulated Other Comprehensive Income | Net unrealized gains and losses on available-for-sale securities
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Net realized investment (losses) gains	(747)	612
Income tax benefit (expense)	261	(92)
Net income	(486)	520
Reclassification from Accumulated Other Comprehensive Income | Net benefit plan assets and obligations recognized in shareholders' equity
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Other underwriting and operating expenses, net	234	474
Income tax benefit (expense)	(81)	(166)
Net income	153	308
Reclassification from Accumulated Other Comprehensive Income | Net unrealized foreign currency translation
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Other revenue	0	(1,467)
Income tax benefit (expense)	0	513
Net income	$ 0	$ (954)