Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net income per consolidated statements of income	$ 108,411	$ 116,038	$ 123,562
Unrealized (losses) gains on available for sale securities and transferred securities:
Unrealized holding (losses) gains arising during the period	(9,667)	(10,309)	10,933
Less: adjustment for net losses (gains) realized in net income	191	0	(185)
Change in net unrealized holding losses on securities transferred to held to maturity	6,070	3,918	3,711
Pension and other postretirement benefit plans:
Net change in prior service costs	154	154	154
Recognized net loss due to BancTrust termination	0	0	837
Recognized net loss due to lump sum settlements	2,412	1,371	559
Change in net actuarial loss	294	2,252	(13,374)
Derivatives:
Change in the accumulated loss on effective cash flow hedge derivatives	(228)	(812)	(1,387)
Less: adjustment for loss realized in net income	370	516	(1)
Other comprehensive (loss) income, net of tax	(404)	(2,910)	1,247
Comprehensive income	$ 108,007	$ 113,128	$ 124,809