Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details)	12 Months Ended
Dec. 31, 2019 USD ($) Investments	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($) Investments
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	$ 92,300,000	$ 281,100,000	$ 406,800,000
Fair Value
Less than 12 Months			90,400,000
12 Months or More			216,900,000
Gross Unrealized Losses
Less than 12 Months			(2,800,000)
12 Months or More			(5,500,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value			307,300,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss			$ (8,300,000)
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities) \| Investments	338		760
Impairment losses	$ 0
Gross Unrealized Losses
Less than 12 Months	(5,400,000)		$ (22,600,000)
12 Months or More	202,500,000		779,000,000.0
12 Months or More	(1,800,000)		(14,500,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,127,900,000		1,902,200,000
Total	(7,200,000)		(37,100,000)
Less than 12 Months	925,400,000		1,123,200,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	1,200,000		189,700,000
Fair Value
Less than 12 Months			45,000,000.0
12 Months or More			120,200,000
Gross Unrealized Losses
Less than 12 Months			(2,200,000)
12 Months or More			(4,300,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value			165,200,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss			(6,500,000)
Gross Unrealized Losses
Less than 12 Months	(4,600,000)		(14,500,000)
12 Months or More	155,900,000		503,700,000
12 Months or More	(1,400,000)		(9,600,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	896,000,000.0		1,238,900,000
Total	(6,000,000.0)		(24,100,000)
Less than 12 Months	740,100,000		735,200,000
US Government Agencies Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	19,800,000		19,800,000
Fair Value
Less than 12 Months			19,500,000
12 Months or More			0
Gross Unrealized Losses
Less than 12 Months			(300,000)
12 Months or More			0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value			19,500,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss			(300,000)
Gross Unrealized Losses
Less than 12 Months	(800,000)		(7,900,000)
12 Months or More	0		154,500,000
12 Months or More	0		(2,300,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	185,300,000		517,600,000
Total	(800,000)		(10,200,000)
Less than 12 Months	185,300,000		363,100,000
Corporate Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	13,900,000		46,300,000
Fair Value
Less than 12 Months			0
12 Months or More			39,600,000
Gross Unrealized Losses
Less than 12 Months			0
12 Months or More			(600,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value			39,600,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss			(600,000)
Gross Unrealized Losses
Less than 12 Months			(200,000)
12 Months or More			51,400,000
12 Months or More			(800,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value			76,300,000
Total			(1,000,000.0)
Less than 12 Months			24,900,000
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Gross Unrealized Losses
Less than 12 Months	0		0
12 Months or More	46,600,000		69,400,000
12 Months or More	(400,000)		(1,800,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	46,600,000		69,400,000
Total	(400,000)		(1,800,000)
Less than 12 Months	0		0
State, county and municipal securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	$ 57,300,000		150,900,000
Fair Value
Less than 12 Months			25,900,000
12 Months or More			57,100,000
Gross Unrealized Losses
Less than 12 Months			(300,000)
12 Months or More			(600,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value			83,000,000.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss			$ (900,000)

Available-for-Sale and Held-to-Maturity, Securities in Unrealized Loss Positions [Abstract]

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Available-for-Sale and Held-to-Maturity, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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