Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) Investments	Dec. 31, 2019 USD ($) Investments
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	$ 51,600,000	$ 92,300,000
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities) | Investments	181	338
Impairment losses	$ 0	$ 0
Less than 12 Months	838,600,000	925,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5,500,000	5,400,000
12 Months or More	16,600,000	202,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	200,000	1,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	855,200,000	1,127,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	5,700,000	7,200,000
Gross Unrealized Losses
Less than 12 Months	(5,500,000)	(5,400,000)
12 Months or More	16,600,000	202,500,000
12 Months or More	(200,000)	(1,800,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	855,200,000	1,127,900,000
Total	(5,700,000)	(7,200,000)
Less than 12 Months	838,600,000	925,400,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	1,000,000.0	1,200,000
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	434,000,000.0	740,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,400,000	4,600,000
12 Months or More	12,300,000	155,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	100,000	1,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	446,300,000	896,000,000.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2,500,000	6,000,000.0
Gross Unrealized Losses
Less than 12 Months	(2,400,000)	(4,600,000)
12 Months or More	12,300,000	155,900,000
12 Months or More	(100,000)	(1,400,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	446,300,000	896,000,000.0
Total	(2,500,000)	(6,000,000.0)
Less than 12 Months	434,000,000.0	740,100,000
US Government Agencies Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity		19,800,000
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	235,600,000	185,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,000,000.0	800,000
12 Months or More	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	235,600,000	185,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2,000,000.0	800,000
Gross Unrealized Losses
Less than 12 Months	(2,000,000.0)	(800,000)
12 Months or More	0	0
12 Months or More	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	235,600,000	185,300,000
Total	(2,000,000.0)	(800,000)
Less than 12 Months	235,600,000	185,300,000
Corporate Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	3,900,000	13,900,000
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	20,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	100,000
12 Months or More	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	20,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	100,000
Gross Unrealized Losses
Less than 12 Months	(100,000)
12 Months or More	0
12 Months or More	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	20,900,000
Total	(100,000)
Less than 12 Months	20,900,000
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
12 Months or More	4,300,000	46,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	100,000	400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4,300,000	46,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	100,000	400,000
Gross Unrealized Losses
Less than 12 Months	0	0
12 Months or More	4,300,000	46,600,000
12 Months or More	(100,000)	(400,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4,300,000	46,600,000
Total	(100,000)	(400,000)
Less than 12 Months	0	0
State, county and municipal securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	46,600,000	$ 57,300,000
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	148,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,000,000.0
12 Months or More	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	148,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1,000,000.0
Gross Unrealized Losses
Less than 12 Months	(1,000,000.0)
12 Months or More	0
12 Months or More	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	148,100,000
Total	(1,000,000.0)
Less than 12 Months	$ 148,100,000