Investment Securities - Additional Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 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	409		409		152
Fair value of investments held to maturity pledged as collateral	$ 1,200,000,000		$ 1,200,000,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years			6 years 9 months
Number of security positions in the securities available for sale portfolio in an unrealized loss position (in security) | security	572		572		327
Fair value of securities available for sale pledged as collateral	$ 1,100,000,000		$ 1,100,000,000
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years			7 years 1 month 14 days
Other than temporary impairment losses recognized in earnings	0	$ 0	$ 0	$ 0
Amortized cost	1,816,527,000		1,816,527,000		$ 1,511,034,000
Available for sale	1,749,001,000		1,749,001,000		$ 1,493,905,000
Losses on securities transactions, net	$ 79,000	$ (6,000)	$ 880,000	$ (5,000)
Four Private Label Mortgage-Backed Securities
Investment Securities [Line Items]
Number of security positions in the securities available for sale portfolio in an unrealized loss position (in security) | security	4		4
Amortized cost	$ 6,800,000		$ 6,800,000
Available for sale	$ 6,200,000		$ 6,200,000
Single Issuer Trust Preferred Securities | Non-Rated
Investment Securities [Line Items]
Number of security positions in the securities held to maturity portfolio in an unrealized loss position (in security) | security	4		4