Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	$ 39,394	$ 86,424	$ 57,852	$ 162,838
Income tax (benefit)	(13,787)	(30,248)	(20,246)	(57,118)
Other comprehensive income, net of tax	25,607	56,176	37,606	105,720
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Net realized investment (losses) gains	(42)	836	(164)	3,892
Other underwriting and operating expenses, net	(38,511)	(35,348)	(79,276)	(75,125)
Other revenue	2,502	3,994	4,924	10,367
Income before tax	180,616	165,239	354,573	268,927
Income tax benefit (expense)	(61,994)	(56,018)	(146,153)	(90,515)
Net income	118,622	109,221	208,420	178,412
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			2,548,842
Other comprehensive income before reclassifications			36,510
Less: Amounts reclassified from AOCL			(1,096)
Balance, end of period	2,995,061	2,515,092	2,995,061	2,515,092
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	(1,172)	364	(1,685)	2,917
Income tax benefit (expense)	409	(127)	589	(898)
Net income	(763)	237	(1,096)	2,019
Accumulated other comprehensive loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive income, net of tax			37,606	105,720
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(75,100)	(60,880)
Balance, end of period	(37,494)	44,840	(37,494)	44,840
Net unrealized gains and losses on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	39,614	86,674	58,261	165,058
Income tax (benefit)	(13,865)	(30,336)	(20,391)	(57,893)
Other comprehensive income, net of tax	25,749	56,338	37,870	107,165
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(20,797)
Other comprehensive income before reclassifications			36,479
Less: Amounts reclassified from AOCL			(1,391)
Balance, end of period	17,073		17,073
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) gains	(1,392)	98	(2,139)	710
Income tax benefit (expense)	487	(34)	748	(126)
Net income	(905)	64	(1,391)	584
Net benefit plan assets and obligations recognized in shareholders' equity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	(220)	(266)	(454)	(740)
Income tax (benefit)	78	93	159	259
Other comprehensive income, net of tax	(142)	(173)	(295)	(481)
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			295
Balance, end of period	(54,567)		(54,567)
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	220	266	454	740
Income tax benefit (expense)	(78)	(93)	(159)	(259)
Net income	142	173	295	481
Net unrealized foreign currency translation
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	0	16	45	(1,480)
Income tax (benefit)	0	(5)	(14)	516
Other comprehensive income, net of tax	0	11	31	(964)
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		0
Net unrealized foreign currency translation | Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Other revenue	0	0	0	1,467
Income tax benefit (expense)	0	0	0	(513)
Net income	$ 0	$ 0	$ 0	$ 954