Investments - Narrative (Details) $ in Thousands	12 Months Ended
Dec. 31, 2020 USD ($) number_of_securities	Dec. 31, 2019 USD ($) number_of_securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 882,682	$ 1,115,323
Other investments	$ 48,764	$ 38,044
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions \| number_of_securities	30	73
Fair Value	$ 912,429	$ 1,135,317
Gross Unrealized Gains	$ 30,104	21,548
Maximum percentage of shareholders' equity security holdings held of one issuer	10.00%
Available-for-sale securities pledged as collateral, fair value	$ 525,800	484,800
Mortgage-backed securities, weighted average life	3 years
accrued interest receivable, held to maturity securities	$ 3,600
Debt Securities, Held-to-maturity, Allowance for Credit Loss	449
Des Moines
Debt Securities, Available-for-sale [Line Items]
Other investments	10,800	15,700
Reclassified to Held to Maturity [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	331,000	116,300
Fair Value	352,600	123,200
Gross Unrealized Gains	$ 25,600	$ 6,600

accrued interest receivable, held to maturity securities

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Maximum percentage of shareholders' equity security holdings held of one issuer.

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Mortgage-backed securities, weighted average life

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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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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity).

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Includes other investments and securities that are not at fair value as of the balance sheet date and those that are not accounted for under the equity method.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions.

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