Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Detail) - USD ($)	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Debt Outstanding [Line Items]
Short-term borrowings	$ 2,631,252,000	$ 1,033,730,000
Long-term debt	$ 696,217,000	695,492,000
Senior Notes \| 3.50% Senior Notes
Debt Outstanding [Line Items]
Maturity	Jan. 29, 2025
Principal value at period end	$ 350,000,000
Long-term debt	$ 347,554,000	$ 347,303,000
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	$ 350,000,000
Long-term debt	$ 348,663,000	$ 348,189,000
Interest rate on senior notes	5.375%	5.375%
Short-term FHLB advances
Debt Outstanding [Line Items]
Principal value at period end	$ 2,250,000,000
Short-term borrowings	2,250,000,000	$ 700,000,000
Federal funds purchased
Debt Outstanding [Line Items]
Principal value at period end	0
Short-term borrowings	0	330,000,000
Securities sold under agreement to repurchase
Debt Outstanding [Line Items]
Principal value at period end	371,539,000
Short-term borrowings	371,539,000	0
Short-term borrowings
Debt Outstanding [Line Items]
Principal value at period end	9,713,000
Short-term borrowings	$ 9,713,000	$ 3,730,000

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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- Definition

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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