Investment Securities Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) Security	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Investments [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 1,600,000		$ 1,700,000
Amortized cost	2,500,245		2,494,059
Available-for-sale Securities, Gross Realized Gain (Loss)	947	$ 4,145
Available-for-sale Securities, Fair Value Disclosure	2,516,205		2,484,773
Single Bank Stock Investment, cost basis	7,400
Single Bank Stock Investment, Fair Value	$ 9,100
Percent Ownership In An Individual Financial Institution	50.30%
IndividualBankStockInvestmentPercenttoTotalPortfolio	10.00%
Auction Rate Certificates Not Subject To Any Additional Other Than Temporary Impairment Charges Fair Value	$ 97,300
Domestic Corporate debt securities	89,900
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	14,228		14,677
Available-for-sale Securities, Gross Realized Gain (Loss)	733	$ 1,970
Available-for-sale Securities, Fair Value Disclosure	19,067		21,514
Equity Securities Financial Institution [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Equity Securities	18,200		20,600
Amortized cost	13,400
Available-for-sale Securities, Fair Value Disclosure	18,200
Equity Securities, Other [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Equity Securities	892
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	106,871		106,772
Available-for-sale Securities, Fair Value Disclosure	97,326		$ 98,059
Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Unrealized Loss on Securities	$ 7,800
Number of Trust Preferred Securities | Security	19
Moody's, A2 Rating [Member] | Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Carrying Value Auction Rate Securities Rated Investment Grade	$ 91,800
Percentage Auction Rate Securities Investment Grade	94.00%
Moody's, A3 Rating [Member] | Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Carrying Value Auction Rate Securities Rated Investment Grade	$ 5,500
Percentage Auction Rate Securities Investment Grade	6.00%
External Credit Rating, BBB [Member] | Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	$ 3,700
Available-for-sale Securities, Fair Value Disclosure	2,400
External Credit Rating, Rated Below Investment Grade [Member] | Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	11,500
Available-for-sale Securities, Fair Value Disclosure	$ 9,000
Number of Trust Preferred Securities | Security	6