Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 229,264	$ 200,274	$ 188,301
Investment securities available for sale
Net unrealized gains (losses)	(12,302)	(17,900)	(37,873)
Net unrealized gain (loss) on available for sale securities transferred to held to maturity securities	(14,738)	0	17,434
Amortization of net unrealized gain (loss) on available for sale securities transferred to held to maturity securities	(2,665)	(5,887)	(555)
Reclassification adjustment for net gain (loss) realized in net income	0	(9,316)	(8,133)
Income tax (expense) benefit	11,331	12,565	11,074
Other comprehensive income (loss) on investment securities available for sale	(18,374)	(20,538)	(18,053)
Defined benefit pension and postretirement obligations
Amortization of prior service cost	(148)	(73)	50
Plan amendments	0	1,759	0
Amortization of actuarial loss (gains)	16,555	(4,026)	(15,636)
Income tax (expense) benefit	(6,112)	815	5,873
Other comprehensive income (loss) on pension and postretirement obligations	10,295	(1,525)	(9,713)
Total other comprehensive income (loss)	(8,079)	(22,063)	(27,766)
Comprehensive income	$ 221,185	$ 178,211	$ 160,535