Investment Securities Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)
Schedule of Investments [Line Items]
Securities pledged as collateral	$ 1,800,000	$ 0
Amortized cost	2,655,314	2,575,719
Available-for-sale Securities, Fair Value Disclosure	2,592,823	2,547,956
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized cost		776
Available-for-sale Securities, Fair Value Disclosure		$ 918
Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Unrealized Loss on Securities	$ 839
Number of Trust Preferred Securities | security	17
External Credit Rating, BBB [Member] | Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	$ 3,800
Available-for-sale Securities, Fair Value Disclosure	$ 3,100
Not Rated By An External Agency [Member] | Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Number of Trust Preferred Securities | security	2
External Credit Rating, Rated Below Investment Grade [Member] | Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	$ 4,900
Available-for-sale Securities, Fair Value Disclosure	$ 4,700
Number of Trust Preferred Securities | security	4
Pooled Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	$ 11,500		$ 11,500