Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Fair Value
Less than 12 Months	$ 1,080,300,000	$ 1,063,400,000
12 Months or More	705,300,000	15,800,000
Total	1,785,600,000	1,079,200,000
Gross Unrealized Losses
Less than 12 Months	(10,700,000)	(19,500,000)
12 Months or More	(13,800,000)	(500,000)
Total	(24,500,000)	(20,000,000)
Fair Value
Less than 12 Months	180,600,000	203,400,000
12 Months or More	87,700,000	22,800,000
Total	268,300,000	226,200,000
Gross Unrealized Losses
Less than 12 Months	(9,700,000)	(8,800,000)
12 Months or More	(3,000,000)	(1,300,000)
Total	$ (12,700,000)	$ (10,100,000)
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities) | security	581	396
Impairment losses	$ 0
Obligations of U.S. government agencies
Fair Value
Less than 12 Months	284,900,000	$ 316,500,000
12 Months or More	266,100,000	0
Total	551,000,000	316,500,000
Gross Unrealized Losses
Less than 12 Months	(3,400,000)	(6,400,000)
12 Months or More	(4,600,000)	0
Total	(8,000,000)	(6,400,000)
Fair Value
Less than 12 Months	9,700,000	19,600,000
12 Months or More	9,900,000	0
Total	19,600,000	19,600,000
Gross Unrealized Losses
Less than 12 Months	0	(200,000)
12 Months or More	(200,000)	0
Total	(200,000)	(200,000)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Fair Value
Less than 12 Months	670,100,000	746,300,000
12 Months or More	439,200,000	15,800,000
Total	1,109,300,000	762,100,000
Gross Unrealized Losses
Less than 12 Months	(6,200,000)	(13,100,000)
12 Months or More	(9,200,000)	(500,000)
Total	(15,400,000)	(13,600,000)
Fair Value
Less than 12 Months	76,400,000	108,800,000
12 Months or More	60,500,000	20,000,000
Total	136,900,000	128,800,000
Gross Unrealized Losses
Less than 12 Months	(9,100,000)	(7,900,000)
12 Months or More	(2,500,000)	(1,300,000)
Total	(11,600,000)	(9,200,000)
Private mortgage-backed securities
Fair Value
Less than 12 Months	74,000,000
12 Months or More	0
Total	74,000,000
Gross Unrealized Losses
Less than 12 Months	(800,000)
12 Months or More	0
Total	(800,000)
Corporate securities
Fair Value
Less than 12 Months	51,300,000
12 Months or More	0
Total	51,300,000
Gross Unrealized Losses
Less than 12 Months	(300,000)
12 Months or More	0
Total	(300,000)
Fair Value
Less than 12 Months	41,200,000	32,500,000
12 Months or More	5,000,000	0
Total	46,200,000	32,500,000
Gross Unrealized Losses
Less than 12 Months	(200,000)	(200,000)
12 Months or More	(100,000)	0
Total	(300,000)	(200,000)
US Treasury notes
Fair Value
Less than 12 Months		600,000
12 Months or More		0
Total		600,000
Gross Unrealized Losses
Less than 12 Months		0
12 Months or More		0
Total		0
State, county and municipal securities
Fair Value
Less than 12 Months	53,300,000	42,500,000
12 Months or More	12,300,000	2,800,000
Total	65,600,000	45,300,000
Gross Unrealized Losses
Less than 12 Months	(400,000)	(500,000)
12 Months or More	(200,000)	0
Total	$ (600,000)	$ (500,000)