Investment Securities - Narrative (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) security	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) security
Investment securities total	$ 1,000,000		$ 1,000,000
Amortized cost	761,037		809,725
Available-for-sale securities	749,537		791,102
Held-to-maturity securities	$ 221,727		$ 235,398
Number of securities | security	183		211
Fair value of available-for-sale securities in an unrealized loss position	$ 616,750		$ 682,892
OTTI charges recorded	$ 0	$ 0
Residential mortgage loans held for sale period	45 days
Fair value of available-for-sale investment securities pledged as collateral	$ 336,200		$ 318,100
Type of available-for-sale investment securities collateral	us-gaap:AssetPledgedAsCollateralMember		us-gaap:AssetPledgedAsCollateralMember
Number of held-to-maturity securities in unrealized loss positions | security	43		49
Fair value of held-to-maturity securities in an unrealized loss position	$ 181,317		$ 229,649
Held-to-maturity investment securities pledged as collateral	$ 128,900		$ 133,100
Type of held-to-maturity investment securities collateral	us-gaap:AssetPledgedAsCollateralMember		us-gaap:AssetPledgedAsCollateralMember
Estimated weighted average expected life of the held-to-maturity mortgage-backed securities portfolio	2 years 8 months 12 days		2 years 9 months 18 days
Life of the held-to-maturity investment portfolio	2 years 6 months		2 years 6 months
Municipal securities maturing within one to five years [Member]
Amortized cost	$ 200
Available-for-sale securities	200
Municipal securities due within five to ten years [Member]
Amortized cost	400
Available-for-sale securities	400
Other Securities Investments with No Contractual Maturity [Member]
Amortized cost	500
Other Securities [Member]
Amortized cost	469		$ 469
Available-for-sale securities	$ 469		$ 469