Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (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
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	$ 1,140,300,000		$ 1,140,300,000		$ 1,080,300,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(26,100,000)		(26,100,000)		(10,700,000)
Available-for-Sale, 12 Months or More, Fair Value	819,500,000		819,500,000		705,300,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(29,800,000)		(29,800,000)		(13,800,000)
Available-for-Sale, Total Fair Value	1,959,800,000		1,959,800,000		1,785,600,000
Available-for-Sale, Total Gross Unrealized Losses	(55,900,000)		(55,900,000)		(24,500,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	186,100,000		186,100,000		180,600,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(14,700,000)		(14,700,000)		(9,700,000)
Held-to-Maturity, 12 Months or More, Fair Value	88,800,000		88,800,000		87,700,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(4,700,000)		(4,700,000)		(3,000,000)
Held-to-Maturity, Total Fair Value	274,900,000		274,900,000		268,300,000
Held-to-Maturity, Total Gross Unrealized Losses	$ (19,400,000)		$ (19,400,000)		$ (12,700,000)
Investment securities in an unrealized loss position (securities) | security	720		720		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	199,900,000		199,900,000		$ 284,900,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(4,800,000)		(4,800,000)		(3,400,000)
Available-for-Sale, 12 Months or More, Fair Value	346,500,000		346,500,000		266,100,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(10,500,000)		(10,500,000)		(4,600,000)
Available-for-Sale, Total Fair Value	546,400,000		546,400,000		551,000,000
Available-for-Sale, Total Gross Unrealized Losses	(15,300,000)		(15,300,000)		(8,000,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	30,900,000		30,900,000		76,400,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(400,000)		(400,000)		(9,100,000)
Held-to-Maturity, 12 Months or More, Fair Value	8,800,000		8,800,000		60,500,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(200,000)		(200,000)		(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	(600,000)		(600,000)		(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	851,800,000		851,800,000		670,100,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(19,200,000)		(19,200,000)		(6,200,000)
Available-for-Sale, 12 Months or More, Fair Value	448,200,000		448,200,000		439,200,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(18,600,000)		(18,600,000)		(9,200,000)
Available-for-Sale, Total Fair Value	1,300,000,000		1,300,000,000		1,109,300,000
Available-for-Sale, Total Gross Unrealized Losses	(37,800,000)		(37,800,000)		(15,400,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	85,800,000		85,800,000		41,200,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(13,100,000)		(13,100,000)		(200,000)
Held-to-Maturity, 12 Months or More, Fair Value	44,000,000		44,000,000		5,000,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(3,700,000)		(3,700,000)		(100,000)
Held-to-Maturity, Total Fair Value	129,800,000		129,800,000		46,200,000
Held-to-Maturity, Total Gross Unrealized Losses	(16,800,000)		(16,800,000)		(300,000)
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	49,900,000		49,900,000		74,000,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(1,200,000)		(1,200,000)		(800,000)
Available-for-Sale, 12 Months or More, Fair Value	22,900,000		22,900,000		0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(600,000)		(600,000)		0
Available-for-Sale, Total Fair Value	72,800,000		72,800,000		74,000,000
Available-for-Sale, Total Gross Unrealized Losses	(1,800,000)		(1,800,000)		(800,000)
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	59,800,000		59,800,000		53,300,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(1,000,000)		(1,000,000)		(400,000)
Held-to-Maturity, 12 Months or More, Fair Value	26,200,000		26,200,000		12,300,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(700,000)		(700,000)		(200,000)
Held-to-Maturity, Total Fair Value	86,000,000		86,000,000		65,600,000
Held-to-Maturity, Total Gross Unrealized Losses	(1,700,000)		(1,700,000)		(600,000)
Corporate securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	38,700,000		38,700,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(900,000)		(900,000)
Available-for-Sale, 12 Months or More, Fair Value	1,900,000		1,900,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(100,000)		(100,000)
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	(200,000)		(200,000)		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	(100,000)		(100,000)		(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)