Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 93,986	$ 72,656	$ 267,833	$ 223,631
Net income, Tax (Expense) Benefit	(36,058)	(25,868)	(100,149)	(81,554)
Net income	57,928	46,788	167,684	142,077
Reclassification adjustment for losses realized in net income, Before-tax Amount	112	112	336	336
Reclassification adjustment for losses realized in net income, Tax (Expense) Benefit	(43)	(43)	(130)	(130)
Reclassification adjustment for losses realized in net income, Net of Tax Amount	69	69	206	206
Net unrealized (losses) gains on investment securities available for sale:
Reclassification adjustment for net gains realized in net income, Before-tax Amount			(2,710)	(1,331)
Net unrealized gains (losses) arising during the period, Before-tax Amount	26,374	(18,173)	12,907	27,467
Net unrealized (losses) gains, Before-tax Amount	26,374	(18,173)	10,197	26,136
Net unrealized (losses) gains on investment securities available for sale: Tax (Expense) Benefit
Reclassification adjustment for net gains realized in net income, Tax (Expense) Benefit			1,043	513
Net unrealized gains (losses) arising during the period, Tax (Expense) Benefit	(10,154)	6,993	(4,966)	(10,579)
Net unrealized (losses) gains, Tax (Expense) Benefit	(10,154)	6,993	(3,923)	(10,066)
Net unrealized (losses) gains on investment securities available for sale: Net of Tax Amount
Reclassification adjustment for net gains realized in net income, Net of Tax Amount			(1,667)	(818)
Net unrealized gains (losses) arising during the period, Net of Tax Amount	16,220	(11,180)	7,941	16,888
Net unrealized (losses) gains, Net of Tax Amount	16,220	(11,180)	6,274	16,070
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for gains realized in net income, Before-tax Amount	(94)	0	(178)	(144)
Actuarial gains arising during the period, Before-tax Amount	0	0	236	395
Net unrealized gains, Before-tax Amount	(94)	0	58	251
Post-retirement unfunded health benefit: Tax (Expense) Benefit
Reclassification adjustment for gains realized in net income, Tax (Expense) Benefit	36	0	68	56
Actuarial gains arising during the period, Tax (Expense) Benefit	0	0	(93)	(152)
Net unrealized gains, Tax (Expense) Benefit	36	0	(25)	(96)
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for gains realized in net income, Net of Tax Amount	(58)	0	(110)	(88)
Actuarial gains arising during the period, Net of Tax Amount	0	0	143	243
Net unrealized gains, Net of Tax Amount	(58)	0	33	155
Other comprehensive income (loss), Before-tax Amount	26,392	(18,061)	10,591	26,723
Other comprehensive income (loss), Tax (Expense) Benefit	(10,161)	6,950	(4,078)	(10,292)
Other comprehensive income (loss), Net of Tax Amount	16,231	(11,111)	6,513	16,431
Comprehensive income, Net of Tax Amount	$ 74,159	$ 35,677	$ 174,197	$ 158,508