Investment Securities Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) Security	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) Security	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Investments [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 1,600,000		$ 1,600,000		$ 1,700,000
Amortized cost	2,438,677		2,438,677		2,312,087
Available-for-sale Securities, Gross Realized Gain (Loss)	2,415	$ 1,112	6,560	$ 1,112
Available-for-sale Securities, Fair Value Disclosure	2,440,492		2,440,492		2,323,371
Single Bank Stock Investment, cost basis	10,700		10,700
Single Bank Stock Investment, Fair Value	$ 15,700		$ 15,700
Percent Ownership In An Individual Financial Institution	57.70%		57.70%
IndividualBankStockInvestmentPercenttoTotalPortfolio	5.00%		5.00%
Auction Rate Certificates Not Subject To Any Additional Other Than Temporary Impairment Charges Fair Value	$ 98,600		$ 98,600
Domestic Corporate debt securities	97,200		97,200
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	23,981		23,981		33,469
Available-for-sale Securities, Gross Realized Gain (Loss)	2,290	$ (12)	4,260	$ (11)
Available-for-sale Securities, Fair Value Disclosure	33,041		33,041		47,623
Equity Securities Financial Institution [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Equity Securities	27,200		27,200		41,800
Amortized cost	18,200		18,200
Available-for-sale Securities, Fair Value Disclosure	27,200		27,200
Equity Securities, Other [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Equity Securities	5,800		5,800
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	106,504		106,504		108,751
Available-for-sale Securities, Fair Value Disclosure	98,606		98,606		$ 100,941
Pooled Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	$ 1,900		1,900
Available-for-sale Securities, Gross Realized Gain (Loss)			$ 2,300
Number of Securities Sold | Security			3
Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Unrealized Loss on Securities			$ 4,200
Number of Trust Preferred Securities | Security	19		19
Number of Trust Preferred Securities Not Rated | Security	3		3
Moody's, A2 Rating [Member] | Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Carrying Value Auction Rate Securities Rated Investment Grade	$ 93,000		$ 93,000
Percentage Auction Rate Securities Investment Grade	94.00%		94.00%
Moody's, A3 Rating [Member] | Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Carrying Value Auction Rate Securities Rated Investment Grade	$ 6,000		$ 6,000
Percentage Auction Rate Securities Investment Grade	6.00%		6.00%
External Credit Rating, BBB [Member] | Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	$ 4,700		$ 4,700
Available-for-sale Securities, Fair Value Disclosure	3,800		3,800
External Credit Rating, Rated Below Investment Grade [Member] | Pooled Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	0		0
Available-for-sale Securities, Fair Value Disclosure	530		530
External Credit Rating, Rated Below Investment Grade [Member] | Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	14,500		14,500
Available-for-sale Securities, Fair Value Disclosure	$ 13,100		$ 13,100
Number of Trust Preferred Securities | Security	7		7