Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income (loss)	$ 17,535	$ (22,592)	$ 75,387	$ 140,246
Income tax (expense) benefit	(6,138)	7,907	(26,384)	(49,211)
Other comprehensive income (loss), net of tax	11,397	(14,685)	49,003	91,035
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Net realized investment (losses) gains	(47)	5,092	(211)	8,984
Other underwriting and operating expenses, net	(39,888)	(37,870)	(119,164)	(112,995)
Other revenue	2,922	3,867	7,846	14,234
Income before tax	184,467	83,749	539,040	352,676
Income tax benefit (expense)	(64,440)	(27,131)	(210,593)	(117,646)
Net income	120,027	56,618	328,447	235,030
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			2,548,842
Other comprehensive income before reclassifications			47,777
Less: Amounts reclassified from AOCL			(1,226)
Balance, end of period	3,130,149	2,583,079	3,130,149	2,583,079
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	(201)	5,618	(1,886)	8,535
Income tax benefit (expense)	71	(1,966)	660	(2,864)
Net income	(130)	3,652	(1,226)	5,671
Accumulated other comprehensive (loss) income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive income (loss), net of tax			49,003	91,035
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(75,100)	(60,880)
Balance, end of period	(26,097)	30,155	(26,097)	30,155
Net unrealized gains and losses on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income (loss)	17,761	(22,206)	76,022	142,852
Income tax (expense) benefit	(6,217)	7,772	(26,608)	(50,121)
Other comprehensive income (loss), net of tax	11,544	(14,434)	49,414	92,731
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(20,797)
Other comprehensive income before reclassifications			47,746
Less: Amounts reclassified from AOCL			(1,668)
Balance, end of period	28,617		28,617
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	(427)	5,248	(2,566)	5,958
Income tax benefit (expense)	150	(1,837)	898	(1,963)
Net income	(277)	3,411	(1,668)	3,995
Net benefit plan assets and obligations recognized in shareholders' equity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income (loss)	(226)	(370)	(680)	(1,110)
Income tax (expense) benefit	79	129	238	388
Other comprehensive income (loss), net of tax	(147)	(241)	(442)	(722)
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			442
Balance, end of period	(54,714)		(54,714)
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	226	370	680	1,110
Income tax benefit (expense)	(79)	(129)	(238)	(388)
Net income	147	241	442	722
Net unrealized foreign currency translation
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income (loss)	0	(16)	45	(1,496)
Income tax (expense) benefit	0	6	(14)	522
Other comprehensive income (loss), net of tax	0	(10)	31	(974)
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