Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) Investments	Dec. 31, 2017 USD ($) Investments
Fair Value
Less than 12 Months	$ 1,123,200,000	$ 1,080,300,000
12 Months or More	779,000,000	705,300,000
Total	1,902,200,000	1,785,600,000
Gross Unrealized Losses
Less than 12 Months	(22,600,000)	(10,700,000)
12 Months or More	(14,500,000)	(13,800,000)
Total	(37,100,000)	(24,500,000)
Fair Value
Less than 12 Months	90,400,000	180,600,000
12 Months or More	216,900,000	87,700,000
Gross Unrealized Losses
Less than 12 Months	(2,800,000)	(9,700,000)
12 Months or More	(5,500,000)	(3,000,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	307,300,000	268,300,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	$ (8,300,000)	$ (12,700,000)
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities) | Investments	760	581
Impairment losses	$ 0
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Fair Value
Less than 12 Months	735,200,000	$ 670,100,000
12 Months or More	503,700,000	439,200,000
Total	1,238,900,000	1,109,300,000
Gross Unrealized Losses
Less than 12 Months	(14,500,000)	(6,200,000)
12 Months or More	(9,600,000)	(9,200,000)
Total	(24,100,000)	(15,400,000)
Fair Value
Less than 12 Months	0	76,400,000
12 Months or More	39,600,000	60,500,000
Gross Unrealized Losses
Less than 12 Months	0	(9,100,000)
12 Months or More	(600,000)	(2,500,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	39,600,000	136,900,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	(600,000)	(11,600,000)
Obligations of U.S. government agencies
Fair Value
Less than 12 Months	363,100,000	284,900,000
12 Months or More	154,500,000	266,100,000
Total	517,600,000	551,000,000
Gross Unrealized Losses
Less than 12 Months	(7,900,000)	(3,400,000)
12 Months or More	(2,300,000)	(4,600,000)
Total	(10,200,000)	(8,000,000)
Fair Value
Less than 12 Months	19,500,000	9,700,000
12 Months or More	0	9,900,000
Gross Unrealized Losses
Less than 12 Months	(300,000)	0
12 Months or More	0	(200,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	19,500,000	19,600,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	(300,000)	(200,000)
Corporate Debt Securities [Member]
Fair Value
Less than 12 Months	24,900,000	51,300,000
12 Months or More	51,400,000	0
Total	76,300,000	51,300,000
Gross Unrealized Losses
Less than 12 Months	(200,000)	(300,000)
12 Months or More	(800,000)	0
Total	(1,000,000)	(300,000)
Fair Value
Less than 12 Months	45,000,000	41,200,000
12 Months or More	120,200,000	5,000,000
Gross Unrealized Losses
Less than 12 Months	(2,200,000)	(200,000)
12 Months or More	(4,300,000)	(100,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	165,200,000	46,200,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	(6,500,000)	(300,000)
Private mortgage-backed securities
Fair Value
Less than 12 Months	0	74,000,000
12 Months or More	69,400,000	0
Total	69,400,000	74,000,000
Gross Unrealized Losses
Less than 12 Months	0	(800,000)
12 Months or More	(1,800,000)	0
Total	(1,800,000)	(800,000)
State, county and municipal securities
Fair Value
Less than 12 Months	25,900,000	53,300,000
12 Months or More	57,100,000	12,300,000
Gross Unrealized Losses
Less than 12 Months	(300,000)	(400,000)
12 Months or More	(600,000)	(200,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	83,000,000	65,600,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	$ (900,000)	$ (600,000)