Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (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
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	$ 1,266,200,000		$ 1,266,200,000		$ 1,080,300,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(26,400,000)		(26,400,000)		(10,700,000)
Available-for-Sale, 12 Months or More, Fair Value	712,200,000		712,200,000		705,300,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(23,500,000)		(23,500,000)		(13,800,000)
Available-for-Sale, Total Fair Value	1,978,400,000		1,978,400,000		1,785,600,000
Available-for-Sale, Total Gross Unrealized Losses	(49,900,000)		(49,900,000)		(24,500,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	194,600,000		194,600,000		180,600,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(13,400,000)		(13,400,000)		(9,700,000)
Held-to-Maturity, 12 Months or More, Fair Value	76,800,000		76,800,000		87,700,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(4,100,000)		(4,100,000)		(3,000,000)
Held-to-Maturity, Total Fair Value	271,400,000		271,400,000		268,300,000
Held-to-Maturity, Total Gross Unrealized Losses	$ (17,500,000)		$ (17,500,000)		$ (12,700,000)
Investment securities in an unrealized loss position (securities) | security	701		701		581
Impairment loss	$ 0	$ 0	$ 0	$ 0
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	266,100,000		266,100,000		$ 284,900,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(6,500,000)		(6,500,000)		(3,400,000)
Available-for-Sale, 12 Months or More, Fair Value	272,100,000		272,100,000		266,100,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(7,300,000)		(7,300,000)		(4,600,000)
Available-for-Sale, Total Fair Value	538,200,000		538,200,000		551,000,000
Available-for-Sale, Total Gross Unrealized Losses	(13,800,000)		(13,800,000)		(8,000,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	34,800,000		34,800,000		76,400,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(600,000)		(600,000)		(9,100,000)
Held-to-Maturity, 12 Months or More, Fair Value	4,900,000		4,900,000		60,500,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(100,000)		(100,000)		(2,500,000)
Held-to-Maturity, Total Fair Value	39,700,000		39,700,000		136,900,000
Held-to-Maturity, Total Gross Unrealized Losses	(700,000)		(700,000)		(11,600,000)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	905,500,000		905,500,000		670,100,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(17,800,000)		(17,800,000)		(6,200,000)
Available-for-Sale, 12 Months or More, Fair Value	415,700,000		415,700,000		439,200,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(15,600,000)		(15,600,000)		(9,200,000)
Available-for-Sale, Total Fair Value	1,321,200,000		1,321,200,000		1,109,300,000
Available-for-Sale, Total Gross Unrealized Losses	(33,400,000)		(33,400,000)		(15,400,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	86,100,000		86,100,000		41,200,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(11,800,000)		(11,800,000)		(200,000)
Held-to-Maturity, 12 Months or More, Fair Value	44,800,000		44,800,000		5,000,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(3,400,000)		(3,400,000)		(100,000)
Held-to-Maturity, Total Fair Value	130,900,000		130,900,000		46,200,000
Held-to-Maturity, Total Gross Unrealized Losses	(15,200,000)		(15,200,000)		(300,000)
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	54,000,000		54,000,000		74,000,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(1,100,000)		(1,100,000)		(800,000)
Available-for-Sale, 12 Months or More, Fair Value	24,400,000		24,400,000		0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(600,000)		(600,000)		0
Available-for-Sale, Total Fair Value	78,400,000		78,400,000		74,000,000
Available-for-Sale, Total Gross Unrealized Losses	(1,700,000)		(1,700,000)		(800,000)
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	64,100,000		64,100,000		53,300,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(900,000)		(900,000)		(400,000)
Held-to-Maturity, 12 Months or More, Fair Value	17,300,000		17,300,000		12,300,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(400,000)		(400,000)		(200,000)
Held-to-Maturity, Total Fair Value	81,400,000		81,400,000		65,600,000
Held-to-Maturity, Total Gross Unrealized Losses	(1,300,000)		(1,300,000)		(600,000)
Corporate securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	40,600,000		40,600,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(1,000,000)		(1,000,000)
Available-for-Sale, 12 Months or More, Fair Value	0		0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	0		0
Available-for-Sale, Total Fair Value	40,600,000		40,600,000
Available-for-Sale, Total Gross Unrealized Losses	(1,000,000)		(1,000,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	9,600,000		9,600,000		9,700,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(100,000)		(100,000)		0
Held-to-Maturity, 12 Months or More, Fair Value	9,800,000		9,800,000		9,900,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(200,000)		(200,000)		(200,000)
Held-to-Maturity, Total Fair Value	19,400,000		19,400,000		19,600,000
Held-to-Maturity, Total Gross Unrealized Losses	$ (300,000)		$ (300,000)		(200,000)
Other Investments
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value					51,300,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses					(300,000)
Available-for-Sale, 12 Months or More, Fair Value					0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses					0
Available-for-Sale, Total Fair Value					51,300,000
Available-for-Sale, Total Gross Unrealized Losses					$ (300,000)