Significant Accounting Policies (Details) $ in Thousands	Mar. 31, 2016 USD ($) Banks ATM branches	Dec. 31, 2015 USD ($)
Basis Of Presentation [Line Items]
Assets	$ 29,171,257	$ 28,792,653
Number of bank divisions | Banks	28
Number of branches | branches	257
Number of ATMs | ATM	335
Cash and Cash Equivalents [Member]
Basis Of Presentation [Line Items]
Total deposits	$ 88	100
Cash and cash equivalents	130,200	117,300
Interest earning deposits with banks	5,500	2,200
Federal funds sold	$ 74,000	$ 65,900