Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 72,656	$ 73,459	$ 223,631	$ 192,918
Net income, Tax (Expense) Benefit	(25,868)	(27,765)	(81,554)	(72,114)
Net income, Net of Tax Amount	46,788	45,694	142,077	120,804
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)	(131)
Reclassification adjustment for losses realized in net income, Net of Tax Amount	69	69	206	205
Net unrealized gains/losses on investment securities available for sale: Before-tax Amount
Reclassification adjustment for net gains realized in net income, Before-tax Amount	0	(1,124)	(1,331)	(2,571)
Net unrealized gains (losses) arising during the period, Before-tax Amount	(18,173)	6,849	27,467	(53,166)
Net unrealized gains (losses), Before-tax Amount	(18,173)	5,725	26,136	(55,737)
Net unrealized gains/losses on investment securities available for sale: Tax (Expense) Benefit
Reclassification adjustment for net gains realized in net income, Tax (Expense) Benefit	0	433	513	990
Net unrealized gains (losses) arising during the period, Tax (Expense) Benefit	6,993	(2,637)	(10,579)	20,468
Net unrealized gains (losses), Tax (Expense) Benefit	6,993	(2,204)	(10,066)	21,458
Net unrealized gains/losses on investment securities available for sale: Net of Tax Amount
Reclassification adjustment for net gains realized in net income, Net of Tax Amount	0	(691)	(818)	(1,581)
Net unrealized gains (losses) arising during the period, Net of Tax Amount	(11,180)	4,212	16,888	(32,698)
Net unrealized gains (losses), Net of Tax Amount	(11,180)	3,521	16,070	(34,279)
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for (gains) losses realized in net income, Before-tax Amount	0	(72)	(144)	(98)
Actuarial gains arising during the period, Before-tax Amount			395	830
Net unrealized gains, Before-tax Amount			251	732
Post-retirement unfunded health benefit: Tax (Expense) Benefit
Reclassification adjustment for (gains) losses realized in net income, Tax (Expense) Benefit	0	28	56	38
Actuarial gains arising during the period, Tax (Expense) Benefit			(152)	(311)
Net unrealized gains, Tax (Expense) Benefit			(96)	(273)
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for (gains) losses realized in net income, Net of Tax Amount	0	(44)	(88)	(60)
Actuarial gains arising during the period, Net of Tax Amount			243	519
Net unrealized gains, Net of Tax Amount			155	459
Other comprehensive (loss) income, Before-tax Amount	(18,061)	5,765	26,723	(54,669)
Other comprehensive (loss) income, Tax (Expense) Benefit	6,950	(2,219)	(10,292)	21,054
Other comprehensive (loss) income, Net of Tax Amount	(11,111)	3,546	16,431	(33,615)
Comprehensive income, Net of Tax Amount	$ 35,677	$ 49,240	$ 158,508	$ 87,189