Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 93,291	$ 134,660	$ 193,522	$ 202,281
Unrealized gains and losses on securities:
Change in unrealized securities gains (losses) arising during period	82,789	13,771	4,155	49,867
Tax effect	(19,042)	(5,125)	(954)	(18,544)
Reclassification adjustment for net gains realized and included in income before income taxes	0	(3,351)	0	(3,327)
Tax effect	0	1,240	0	1,231
Total change in unrealized gains and losses on securities, net of tax	63,747	6,535	3,201	29,227
Unrealized losses on securities available for sale transferred to held to maturity
Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity Arising During Period Before Tax	(109,507)	0	(109,507)	0
Tax effect	25,186	0	25,186	0
Reclassification Adjustment For Accretion On Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity Arising During Period Before Tax	4,473	0	4,473	0
Tax effect	(1,028)	0	(1,028)	0
Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity Arising During Period Net Of Tax	(80,876)	0	(80,876)	0
Change in pension obligation:
Amortization of actuarial losses and prior service cost	3,654	2,460	6,991	4,960
Tax effect	(840)	(897)	(1,608)	(1,838)
Total change in pension obligation, net of tax	2,814	1,563	5,383	3,122
Net current period other comprehensive (loss) income	(14,315)	8,098	(72,292)	32,349
Total comprehensive income	$ 78,976	$ 142,758	$ 121,230	$ 234,630