Loans and Allowance for Loan Losses (Narrative) (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($) Contract	Jun. 30, 2015 USD ($) Contract	Jun. 30, 2016 USD ($) Contract	Jun. 30, 2015 USD ($) Contract	Dec. 31, 2015 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average recorded investment in impaired loans	$ 281,904,000	$ 375,500,000	$ 290,257,000	$ 401,500,000	$ 358,254,000
Interest income recognized	2,046,000		4,029,000	0	9,522,000
Interest income recognized for accruing TDRs		$ 2,500,000		$ 5,100,000
Non-accrual status	$ 65,400,000		$ 65,400,000		77,900,000
Number of contracts | Contract	1	2	1	2
Allowance for loan losses	$ 255,076,000	$ 254,702,000	$ 255,076,000	$ 254,702,000	252,496,000	$ 254,516,000	$ 253,371,000	$ 261,317,000
Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retail loan substandard period (in days)			90 days
Loss And Charged Off [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retail loans downgraded to loss (in days)			120 days
Accruing TDRs With Modifications And Renewals Completed [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accruing troubled debt restructuring	205,200,000		$ 205,200,000		223,900,000
Allowance for loan losses	$ 12,700,000		12,700,000		$ 12,600,000
Accruing TDRs With Modifications And Renewals Completed [Member] | Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial-type impaired loans			1,000,000
Troubled Debt Restructurings That Subsequently Defaulted [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Post-default balance for modifications classified as TDRs		$ 0	$ 92,000	$ 115,000