Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 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	$ 687.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	5.7	$ 7.2
Available-for-sale Securities, Gross Realized Gains	0.0
Available-for-sale Securities, Gross Realized Losses		0.0
Available-for-Sale:
Amortized Cost	3,932.7	2,945.5
Gross Unrealized Gains	81.7	21.7
Gross Unrealized Losses	(5.7)	(7.2)
Estimated Fair Value	4,008.7	2,960.0
Held-to-Maturity:
Amortized Cost	51.6	92.3
Gross Unrealized Gains	3.4	2.2
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	55.0	94.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	991.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	8.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	8.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,587.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	1,362.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	27.8
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	29.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	736.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	996.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	14.1
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	16.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	921.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	658.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	1.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	1.4
US Treasury notes
Available-for-Sale:
Amortized Cost		9.0
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Estimated Fair Value		9.0
State, county and municipal securities
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	462.1
Gross Unrealized Gains	4.8	0.8
Gross Unrealized Losses	(1.0)	0.0
Estimated Fair Value	465.9	80.9
Held-to-Maturity:
Amortized Cost	46.6	57.3
Gross Unrealized Gains	3.2	2.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	49.8	59.4
Available-for-sale securities, amortized cost basis	38.6	80.1
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	2.0	0.8
Available-for-Sale:
Amortized Cost	332.9	367.5
Gross Unrealized Gains	1.0	0.1
Gross Unrealized Losses	(2.0)	(0.8)
Estimated Fair Value	331.9	366.8
Held-to-Maturity:
Amortized Cost		19.8
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Estimated Fair Value		19.8
Corporate Debt Securities [Member]
Schedule of Available for Sale and Held-to-Maturity Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0.1
Available-for-Sale:
Amortized Cost	295.8	134.5
Gross Unrealized Gains	6.5	1.2
Gross Unrealized Losses	(0.1)	0.0
Estimated Fair Value	302.2	135.7
Held-to-Maturity:
Amortized Cost	3.9	13.9
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	4.0	14.0
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	2.5	6.0
Available-for-Sale:
Amortized Cost	2,830.8	2,303.6
Gross Unrealized Gains	69.3	19.6
Gross Unrealized Losses	(2.5)	(6.0)
Estimated Fair Value	2,897.6	2,317.2
Held-to-Maturity:
Amortized Cost	1.0	1.2
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	1.1	1.2
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	0.1	0.4
Available-for-Sale:
Amortized Cost	10.9	47.6
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(0.1)	(0.4)
Estimated Fair Value	10.9	47.2
Other Debt Obligations [Member]
Available-for-Sale:
Amortized Cost	0.2	3.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	0.2	3.2
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	26.0
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	282.4
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value	281.7
Held-to-Maturity:
Investment Securities, Callable Structured, Amortized Cost Basis	$ 0.0