Investment Securities - Additional Information (Details)	3 Months Ended		9 Months Ended
	Jun. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) security
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position (in security) | security			82		378
Fair value of investments held to maturity pledged as collateral			$ 1,400,000,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years			5 years 1 month 6 days
Number of security positions in the securities available for sale portfolio in an unrealized loss position (in security) | security			177		545
Fair value of securities available for sale pledged as collateral			$ 1,100,000,000
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years			6 years 1 month 6 days
Other than temporary impairment losses recognized in earnings	$ 2,900,000	$ 0	$ 2,900,000	$ 0
Amortized cost			1,615,073,000		$ 1,796,404,000
Fair Value			1,628,062,000		1,749,544,000
Gross unrealized losses			$ 3,921,000		$ 48,855,000
Non-Rated Single Issuer Trust Preferred Securities
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position (in security) | security			4
US States and Political Subdivisions Debt Securities, Special Revenue Bonds, Impaired Securities
Investment Securities [Line Items]
Amortized cost			$ 680,000
Fair Value			680,000
US States and Political Subdivisions Debt Securities, Special Revenue Bonds
Investment Securities [Line Items]
Amortized cost			300,800,000
Fair Value			304,500,000
Gross unrealized losses			$ 757,000