Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	$ 1,399,800,000		$ 1,080,300,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(27,000,000)		(10,700,000)
Available-for-Sale, 12 Months or More, Fair Value	648,100,000		705,300,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(19,300,000)		(13,800,000)
Available-for-Sale, Total Fair Value	2,047,900,000		1,785,600,000
Available-for-Sale, Total Gross Unrealized Losses	(46,300,000)		(24,500,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	214,000,000		180,600,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(12,700,000)		(9,700,000)
Held-to-Maturity, 12 Months or More, Fair Value	73,700,000		87,700,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(3,700,000)		(3,000,000)
Held-to-Maturity, Total Fair Value	287,700,000		268,300,000
Held-to-Maturity, Total Gross Unrealized Losses	$ (16,400,000)		$ (12,700,000)
Investment securities in an unrealized loss position (securities) | security	707		581
Impairment loss	$ 0	$ 0
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	310,100,000		$ 284,900,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(7,000,000)		(3,400,000)
Available-for-Sale, 12 Months or More, Fair Value	239,700,000		266,100,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(6,000,000)		(4,600,000)
Available-for-Sale, Total Fair Value	549,800,000		551,000,000
Available-for-Sale, Total Gross Unrealized Losses	(13,000,000)		(8,000,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	44,800,000		76,400,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(600,000)		(9,100,000)
Held-to-Maturity, 12 Months or More, Fair Value	4,900,000		60,500,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(100,000)		(2,500,000)
Held-to-Maturity, Total Fair Value	49,700,000		136,900,000
Held-to-Maturity, Total Gross Unrealized Losses	(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	954,200,000		670,100,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(17,500,000)		(6,200,000)
Available-for-Sale, 12 Months or More, Fair Value	408,400,000		439,200,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(13,300,000)		(9,200,000)
Available-for-Sale, Total Fair Value	1,362,600,000		1,109,300,000
Available-for-Sale, Total Gross Unrealized Losses	(30,800,000)		(15,400,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	85,600,000		41,200,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(11,000,000)		(200,000)
Held-to-Maturity, 12 Months or More, Fair Value	47,200,000		5,000,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(3,100,000)		(100,000)
Held-to-Maturity, Total Fair Value	132,800,000		46,200,000
Held-to-Maturity, Total Gross Unrealized Losses	(14,100,000)		(300,000)
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	74,700,000		74,000,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(1,500,000)		(800,000)
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	74,700,000		74,000,000
Available-for-Sale, Total Gross Unrealized Losses	(1,500,000)		(800,000)
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	74,000,000		53,300,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(1,000,000)		(400,000)
Held-to-Maturity, 12 Months or More, Fair Value	11,700,000		12,300,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(300,000)		(200,000)
Held-to-Maturity, Total Fair Value	85,700,000		65,600,000
Held-to-Maturity, Total Gross Unrealized Losses	(1,300,000)		(600,000)
Corporate securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	60,800,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(1,000,000)
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	60,800,000
Available-for-Sale, Total Gross Unrealized Losses	(1,000,000)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months, Fair Value	9,600,000		9,700,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses	(100,000)		0
Held-to-Maturity, 12 Months or More, Fair Value	9,900,000		9,900,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses	(200,000)		(200,000)
Held-to-Maturity, Total Fair Value	19,500,000		19,600,000
Held-to-Maturity, Total Gross Unrealized Losses	$ (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)