Loans and Credit Quality Loans and Credit Quality (Details 6) $ in Thousands	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jun. 30, 2014 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 12,034,189		$ 11,882,432
NonCovered [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	11,811,551		11,591,641	$ 10,791,669
Oil and Gas Properties [Member] | Lease Agreements [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 90,189	$ 92,865
Financing Receivable, Number Of Contracts	21	21
Oil and Gas Properties [Member] | Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 87,005	$ 88,585
Financing Receivable, Number Of Contracts	8	9
Oil and Gas Properties [Member] | Loans and Leases [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 177,194	$ 181,450
Financing Receivable, Number Of Contracts	29	30
Classified [Member] | NonCovered [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 379,988		$ 242,611
Classified [Member] | Oil and Gas Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 64,232	$ 65,498
Financing Receivable, Number Of Contracts	4	5
Nonperforming Financing Receivable [Member] | Oil and Gas Properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 64,232	$ 65,074
Financing Receivable, Number Of Contracts	4	4