Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available for Sale and Held-to-Maturity Securities
Available-for-sale Securities, Gross Realized Gains	$ 0.0	$ 1.1	$ 0.4
Available-for-sale Securities, Gross Realized Losses	(0.1)	(0.4)	$ (0.1)
Available-for-Sale:
Amortized Cost	2,305.2	2,229.3
Gross Unrealized Gains	2.6	3.9
Gross Unrealized Losses	(37.1)	(24.5)
Estimated Fair Value	2,270.7	2,208.7
Held-to-Maturity:
Amortized Cost	406.8	484.5
Gross Unrealized Gains	2.2	11.5
Gross Unrealized Losses	(8.3)	(12.7)
Estimated Fair Value	400.7	483.3
US Treasury notes
Available-for-Sale:
Amortized Cost	2.6	3.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	2.6	3.2
State, county and municipal securities
Held-to-Maturity:
Amortized Cost	150.9	172.4
Gross Unrealized Gains	1.8	2.6
Gross Unrealized Losses	(0.9)	(0.6)
Estimated Fair Value	151.8	174.4
Available-for-sale securities, amortized cost basis	115.5
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	569.3	569.5
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(10.2)	(8.0)
Estimated Fair Value	559.2	561.5
Held-to-Maturity:
Amortized Cost	19.8	19.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.3)	(0.2)
Estimated Fair Value	19.5	19.6
Corporate Debt Securities [Member]
Available-for-Sale:
Amortized Cost	92.9	88.0
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	(1.0)	(0.3)
Estimated Fair Value	91.9	87.8
Held-to-Maturity:
Amortized Cost	46.3	61.6
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	(0.6)	(0.3)
Estimated Fair Value	45.8	61.4
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	1,566.4	1,474.1
Gross Unrealized Gains	2.5	3.8
Gross Unrealized Losses	(24.1)	(15.4)
Estimated Fair Value	1,544.8	1,462.5
Held-to-Maturity:
Amortized Cost	189.7	230.5
Gross Unrealized Gains	0.3	8.8
Gross Unrealized Losses	(6.5)	(11.6)
Estimated Fair Value	183.5	227.7
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	72.0	91.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(1.8)	(0.8)
Estimated Fair Value	70.2	90.7
Other Investments
Available-for-Sale:
Amortized Cost	2.0	3.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	2.0	3.0
Other Investments
Held-to-Maturity:
Amortized Cost	0.1	0.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	0.1	$ 0.2
Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming \| State, county and municipal securities
Held-to-Maturity:
Available-for-sale securities, amortized cost basis	$ 66.8

Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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