Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) Investments	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) Investments	Dec. 31, 2017 USD ($)
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	$ 872,700,000			$ 1,123,200,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(9,700,000)			(22,600,000)
Available-for-Sale, 12 Months or More, Fair Value	697,600,000			779,000,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(8,000,000)			(14,500,000)
Available-for-Sale, Total Fair Value	1,570,300,000			1,902,200,000
Available-for-Sale, Total Gross Unrealized Losses	$ (17,700,000)			(37,100,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value			$ 28,600,000	90,400,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses			(300,000)	(2,800,000)
Held-to-Maturity, 12 Months or More, Fair Value			44,600,000	216,900,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses			(200,000)	(5,500,000)
Held-to-Maturity, Total Fair Value			73,200,000	307,300,000
Held-to-Maturity, Total Gross Unrealized Losses			$ (500,000)	(8,300,000)
Investment securities in an unrealized loss position (securities) | Investments	551		760
Impairment loss	$ 0	$ 0
Available-for-sale Securities, Gross Realized Gain (Loss)	0	$ 0
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	336,800,000			363,100,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(2,800,000)			(7,900,000)
Available-for-Sale, 12 Months or More, Fair Value	129,700,000			154,500,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(1,200,000)			(2,300,000)
Available-for-Sale, Total Fair Value	466,500,000			517,600,000
Available-for-Sale, Total Gross Unrealized Losses	(4,000,000)			(10,200,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value			$ 0	19,500,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses			0	(300,000)
Held-to-Maturity, 12 Months or More, Fair Value			28,000,000	0
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses			(100,000)	0
Held-to-Maturity, Total Fair Value			28,000,000	19,500,000
Held-to-Maturity, Total Gross Unrealized Losses			(100,000)	(300,000)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	535,900,000			735,200,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(6,900,000)			(14,500,000)
Available-for-Sale, 12 Months or More, Fair Value	450,300,000			503,700,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(5,500,000)			(9,600,000)
Available-for-Sale, Total Fair Value	986,200,000			1,238,900,000
Available-for-Sale, Total Gross Unrealized Losses	(12,400,000)			(24,100,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value				45,000,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses				(2,200,000)
Held-to-Maturity, 12 Months or More, Fair Value				120,200,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses				(4,300,000)
Held-to-Maturity, Total Fair Value				165,200,000
Held-to-Maturity, Total Gross Unrealized Losses				(6,500,000)
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	0			0
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	0			0
Available-for-Sale, 12 Months or More, Fair Value	64,500,000			69,400,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(900,000)			(1,800,000)
Available-for-Sale, Total Fair Value	64,500,000			69,400,000
Available-for-Sale, Total Gross Unrealized Losses	(900,000)			(1,800,000)
State, county and municipal securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	0
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	0
Available-for-Sale, 12 Months or More, Fair Value	16,200,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(100,000)
Available-for-Sale, Total Fair Value	16,200,000
Available-for-Sale, Total Gross Unrealized Losses	(100,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value			9,000,000	25,900,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses			(100,000)	(300,000)
Held-to-Maturity, 12 Months or More, Fair Value			16,600,000	57,100,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses			(100,000)	(600,000)
Held-to-Maturity, Total Fair Value			25,600,000	83,000,000
Held-to-Maturity, Total Gross Unrealized Losses			(200,000)	(900,000)
Corporate securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	0			24,900,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	0			(200,000)
Available-for-Sale, 12 Months or More, Fair Value	36,900,000			51,400,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(300,000)			(800,000)
Available-for-Sale, Total Fair Value	36,900,000			76,300,000
Available-for-Sale, Total Gross Unrealized Losses	$ (300,000)			(1,000,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value			19,600,000	0
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses			(200,000)	0
Held-to-Maturity, 12 Months or More, Fair Value			0	39,600,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses			0	(600,000)
Held-to-Maturity, Total Fair Value			19,600,000	39,600,000
Held-to-Maturity, Total Gross Unrealized Losses			$ (200,000)	$ (600,000)