Investment Securities Narrative (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($) Security	Dec. 31, 2015 USD ($)
Schedule of Investments [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 1,800,000	$ 1,700,000
Amortized cost	2,477,938	2,494,059
Available-for-sale Securities, Fair Value Disclosure	2,508,068	2,484,773
Single Bank Stock Investment, cost basis	7,400
Single Bank Stock Investment, Fair Value	$ 11,400
Percent Ownership In An Individual Financial Institution	52.70%
Individual Bank Stock Investment Percent to Total Portfolio	10.00%
Auction Rate Certificates Not Subject To Any Additional Other Than Temporary Impairment Charges Fair Value	$ 97,700
Domestic Corporate debt securities	107,200
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	14,206	14,677
Available-for-sale Securities, Fair Value Disclosure	22,483	21,514
Equity Securities Financial Institution [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Equity Securities	21,600	20,600
Amortized cost	13,400
Available-for-sale Securities, Fair Value Disclosure	21,600
Equity Securities, Other [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Equity Securities	901
Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Unrealized Loss on Securities	$ 4,500
Number of Trust Preferred Securities | Security	19
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	$ 10,100
Number of Trust Preferred Securities | Security	6
Fair Value, Measurements, Recurring [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Fair Value Disclosure	$ 2,508,068	2,484,773
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Fair Value Disclosure	22,483	21,514
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Fair Value Disclosure	22,483	21,514
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Fair Value Disclosure	22,483	21,514
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities Financial Institution [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Equity Securities	21,600	20,600
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities, Other [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Equity Securities	$ 901	$ 914