Investment Securities - Additional Information (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($)	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	362		362		152
Fair value of investments held to maturity pledged as collateral	$ 1,100,000,000		$ 1,100,000,000
Weighted-average remaining expected life of residential mortgage-backed securities held to maturity, years			8 years 5 months
Number of security positions in the securities available for sale portfolio in an unrealized loss position (in security) | security	567		567		327
Amortized cost	$ 1,888,953,000		$ 1,888,953,000		$ 1,511,034,000
Available for sale	1,833,467,000		1,833,467,000		$ 1,493,905,000
Fair value of securities available for sale pledged as collateral	1,000,000,000		$ 1,000,000,000
Weighted-average remaining expected life of residential mortgage-backed securities available for sale, years			8 years 5 months
Other than temporary impairment losses recognized in earnings	0	$ 0	$ 0	$ 0
(Losses) gains on securities transactions, net	(36,000)	$ 22,000	(801,000)	$ (1,000)
Four Private Label Mortgage-Backed Securities
Investment Securities [Line Items]
Amortized cost	7,000,000		7,000,000
Available for sale	$ 6,400,000		$ 6,400,000
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