Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Debt Outstanding [Line Items]
Short-term borrowings	$ 14,455	$ 631,412
Long-term debt	$ 696,715	696,465
Senior Notes \| 3.50% Senior Notes
Debt Outstanding [Line Items]
Maturity	Jan. 29, 2025
Principal value at period end	$ 350,000
Long-term debt	$ 347,725	$ 347,639
Interest rate on senior notes	3.50%	3.50%
Senior Notes \| 5.375% Senior Notes
Debt Outstanding [Line Items]
Maturity	Sep. 15, 2020
Principal value at period end	$ 350,000
Long-term debt	$ 348,990	$ 348,826
Interest rate on senior notes	5.375%
Short-term FHLB advances
Debt Outstanding [Line Items]
Short-term borrowings	$ 0	$ 300,000
Securities sold under agreement to repurchase
Debt Outstanding [Line Items]
Short-term borrowings	0	319,414
Short-term borrowings
Debt Outstanding [Line Items]
Principal value at period end	14,455
Short-term borrowings	$ 14,455	$ 11,998

Debt Outstanding [Line Items]

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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