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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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Amount of realized loss 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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Amount of realized gain (loss) on investment.

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Investment Securities - Realized Gains (Losses) on Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	$ 4,859.3	$ 3,932.7
Realized Investment Gains (Losses)		0.0	$ 0.0
Debt Securities, Available-for-sale, Realized Gain	3.2
Debt Securities, Available-for-sale, Realized Loss	2.1
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	356.0	332.9
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	2,027.3	2,830.8
State, county and municipal securities
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	434.7	462.1
State, county and municipal securities \| Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	40.7
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	174.4	10.9
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	271.1	295.8
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost		0.2
Mortgage Backed Securities, Variable Rate
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	$ 84.8	$ 220.7