Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Schedule of Available for Sale and Held-to-Maturity Securities
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost	$ 677.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	7.2	$ 37.1
Available-for-sale Securities, Gross Realized Gains	0.0		$ 1.1
Available-for-sale Securities, Gross Realized Losses		0.0	$ (0.4)
Available-for-Sale:
Amortized Cost	2,945.5	2,305.2
Gross Unrealized Gains	21.7	2.6
Gross Unrealized Losses	(7.2)	(37.1)
Estimated Fair Value	2,960.0	2,270.7
Held-to-Maturity:
Amortized Cost	92.3	406.8		$ 281.1
Gross Unrealized Gains	2.2	2.2
Gross Unrealized Losses	0.0	(8.3)
Estimated Fair Value	94.5	400.7		$ 275.1
Debt Securities, Unrealized Gain (Loss)		6.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	661.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	38.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	38.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,486.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	1,470.9
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	29.7
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	30.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	328.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	428.2
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	21.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	23.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	454.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	399.5
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	2.7
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	2.7
US Treasury notes
Available-for-Sale:
Amortized Cost	9.0	2.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	9.0	2.6
State, county and municipal securities
Available-for-Sale:
Amortized Cost	80.1
Gross Unrealized Gains	0.8
Gross Unrealized Losses	0.0
Estimated Fair Value	80.9
Held-to-Maturity:
Amortized Cost	57.3	150.9
Gross Unrealized Gains	2.1	1.8
Gross Unrealized Losses	0.0	(0.9)
Estimated Fair Value	59.4	151.8
Available-for-sale securities, amortized cost basis	45.9
US Government Agencies Debt Securities [Member]
Schedule of Available for Sale and Held-to-Maturity Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0.8	10.2
Available-for-Sale:
Amortized Cost	367.5	569.3
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	(0.8)	(10.2)
Estimated Fair Value	366.8	559.2
Held-to-Maturity:
Amortized Cost	19.8	19.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.3)
Estimated Fair Value	19.8	19.5
Corporate Debt Securities [Member]
Schedule of Available for Sale and Held-to-Maturity Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		1.0
Available-for-Sale:
Amortized Cost	134.5	92.9
Gross Unrealized Gains	1.2	0.0
Gross Unrealized Losses	0.0	(1.0)
Estimated Fair Value	135.7	91.9
Held-to-Maturity:
Amortized Cost	13.9	46.3
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0.0	(0.6)
Estimated Fair Value	14.0	45.8
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available for Sale and Held-to-Maturity Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	6.0	24.1
Available-for-Sale:
Amortized Cost	2,303.6	1,566.4
Gross Unrealized Gains	19.6	2.5
Gross Unrealized Losses	(6.0)	(24.1)
Estimated Fair Value	2,317.2	1,544.8
Held-to-Maturity:
Amortized Cost	1.2	189.7
Gross Unrealized Gains	0.0	0.3
Gross Unrealized Losses	0.0	(6.5)
Estimated Fair Value	1.2	183.5
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available for Sale and Held-to-Maturity Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0.4	1.8
Available-for-Sale:
Amortized Cost	47.6	72.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.4)	(1.8)
Estimated Fair Value	47.2	70.2
Other Debt Obligations [Member]
Available-for-Sale:
Amortized Cost	3.2	2.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	3.2	2.0
Other Investments
Held-to-Maturity:
Amortized Cost	0.1	0.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	0.1	$ 0.1
Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming | State, county and municipal securities
Held-to-Maturity:
Available-for-sale securities, amortized cost basis	28.0
Callable Within One Year [Member]
Schedule of Available for Sale and Held-to-Maturity Securities
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	181.2
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value	181.1
Held-to-Maturity:
Investment Securities, Callable Structured, Amortized Cost Basis	$ 0.0