Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Components of Other Comprehensive Income (Loss) [Abstract]
Net unrealized holding gains (losses) arising during the period	$ 86,674	$ (64,118)	$ 165,058	$ (44,397)
Income tax (expense) benefit	(30,336)	22,362	(57,893)	15,486
Valuation allowance	0	(21,890)	0	(15,172)
Net of taxes	56,338	(63,646)	107,165	(44,083)
Net changes in benefit plan assets and obligations	(266)	(392)	(740)	(1,092)
Income tax benefit	93	137	259	382
Valuation allowance	0	(137)	0	(382)
Net of taxes	(173)	(392)	(481)	(1,092)
Net changes in unrealized foreign currency translation adjustment	16	598	(1,480)	(2,504)
Income tax (expense) benefit	(5)	(208)	516	880
Net of taxes	11	390	(964)	(1,624)
Total other comprehensive income (loss)	86,424	(63,912)	162,838	(47,993)
Total income tax (expense) benefit, net of valuation allowance	(30,248)	264	(57,118)	1,194
Other comprehensive income (loss), net of tax	56,176	(63,648)	105,720	(46,799)
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Reclassification adjustment	165,239	114,976	268,927	251,437
Total income tax expense, net of valuation allowance	(56,018)	(1,322)	(90,515)	(4,707)
Valuation allowance	0	38,669	0	83,167
Net income	109,221	113,654	178,412	246,730
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			2,236,140
Balance, end of period	2,511,679	1,238,284	2,511,679	1,238,284
Accumulated other comprehensive income (loss)
Components of Other Comprehensive Income (Loss) [Abstract]
Other comprehensive income (loss), net of tax			105,720	(46,799)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(60,880)	(81,341)
Other comprehensive income (loss) before reclassifications			107,739
Less: Amounts reclassified from accumulated other comprehensive income (loss)			2,019
Balance, end of period	44,840	(128,140)	44,840	(128,140)
Net unrealized gains and losses on available-for-sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(17,148)
Other comprehensive income (loss) before reclassifications			107,749
Less: Amounts reclassified from accumulated other comprehensive income (loss)			584
Balance, end of period	90,017		90,017
Net benefit plan assets and obligations recognized in shareholders' equity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(44,652)
Other comprehensive income (loss) before reclassifications			0
Less: Amounts reclassified from accumulated other comprehensive income (loss)			481
Balance, end of period	(45,133)		(45,133)
Net unrealized foreign currency translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			920
Other comprehensive income (loss) before reclassifications			(10)
Less: Amounts reclassified from accumulated other comprehensive income (loss)			954
Balance, end of period	(44)		(44)
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Reclassification adjustment	364	869	2,917	12,803
Total income tax expense, net of valuation allowance	(127)	(39)	(898)	(44)
Net income	237	830	2,019	12,759
Reclassification from Accumulated Other Comprehensive Income | Net unrealized gains and losses on available-for-sale securities
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Reclassification adjustment	98	477	710	11,711
Total income tax expense, net of valuation allowance	(34)	(161)	(126)	(4,092)
Valuation allowance	0	122	0	4,048
Net income	64	438	584	11,667
Reclassification from Accumulated Other Comprehensive Income | Net benefit plan assets and obligations recognized in shareholders' equity
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Reclassification adjustment	266	392	740	1,092
Total income tax expense, net of valuation allowance	(93)	(137)	(259)	(382)
Valuation allowance	0	137	0	382
Net income	173	392	481	1,092
Reclassification from Accumulated Other Comprehensive Income | Net unrealized foreign currency translation
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Reclassification adjustment	0	0	1,467	0
Total income tax expense, net of valuation allowance	0	0	(513)	0
Net income	$ 0	$ 0	$ 954	$ 0