Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 83,730	$ 85,812
Net income, Tax (Expense) Benefit	(31,199)	(31,849)
Net income	52,531	53,963
Reclassification adjustment for losses realized in net income, Before-tax Amount	273	112
Reclassification adjustment for losses realized in net income, Tax (Expense) Benefit	(105)	(43)
Reclassification adjustment for losses realized in net income, Net of Tax Amount	168	69
Net unrealized gains on investment securities available for sale: Before-tax Amount
Reclassification adjustment for net gains realized in net income, Before-tax Amount	(67)	(725)
Net unrealized gains arising during the period, Before-tax Amount	47,172	15,211
Net unrealized gains, Before-tax Amount	47,105	14,486
Net unrealized gains on investment securities available for sale: Tax (Expense) Benefit
Reclassification adjustment for net gains realized in net income, Tax (Expense) Benefit	26	281
Net unrealized gains arising during the period, Tax (Expense) Benefit	(18,162)	(5,856)
Net unrealized gains, Tax (Expense) Benefit	(18,136)	(5,575)
Net unrealized gains on investment securities available for sale: Net of Tax Amount
Reclassification adjustment for net gains realized in net income, Net of Tax Amount	(41)	(444)
Net unrealized gains arising during the period, Net of Tax Amount	29,010	9,355
Net unrealized gains, Net of Tax Amount	28,969	8,911
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for gains realized in net income, Before-tax Amount	(94)	(42)
Net unrealized gains, Before-tax Amount	(94)	(42)
Post-retirement unfunded health benefit: Tax (Expense) Benefit
Reclassification adjustment for gains realized in net income, Tax (Expense) Benefit	36	16
Net unrealized gains, Tax (Expense) Benefit	36	16
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for gains realized in net income, Net of Tax Amount	(58)	(26)
Net unrealized gains, Net of Tax Amount	(58)	(26)
Other comprehensive income, Before-tax Amount	47,284	14,556
Other comprehensive income, Tax (Expense) Benefit	(18,205)	(5,602)
Other comprehensive income, Net of Tax Amount	29,079	8,954
Comprehensive income, Net of Tax Amount	$ 81,610	$ 62,917