Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 116,038	$ 123,562	$ 117,060
Unrealized (losses) gains on available for sale securities and transferred securities:
Unrealized holding (losses) gains arising during the period	(10,309)	10,933	(41,456)
Less: adjustment for net gains realized in net income	0	(185)	(299)
Change in net unrealized holding losses on securities transferred to held to maturity	3,918	3,711	(28,642)
Pension and other postretirement benefit plans:
Net change in prior service costs	154	154	155
Recognized net loss due to BancTrust termination		837	0
Recognized net loss due to lump sum settlements	1,371	559	1,374
Change in net actuarial loss	2,252	(13,374)	20,218
Derivatives:
Change in the accumulated loss on effective cash flow hedge derivatives	(812)	(1,387)	1,524
Less: adjustment for loss realized in net income	516	(1)	0
Other comprehensive (loss) income, net of tax	(2,910)	1,247	(47,126)
Comprehensive income	$ 113,128	$ 124,809	$ 69,934