Short-Term Debt - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Committed line of credit	$ 10,000,000	$ 10,000,000
Securities pledged as collateral	812,000,000	812,000,000
Average balance of short-term debt	1,870,000,000	1,400,000,000
Accrued interest payable on short-term debt	1,610,000	1,610,000	1,000,000
Committed line of credit with financial institutions, outstanding	0	0	0
Residential Loans
Short-term Debt [Line Items]
Loan pledged as collateral	1,320,000,000	1,320,000,000
Commercial Loans
Short-term Debt [Line Items]
Committed line of credit	100,000,000	100,000,000
Loan pledged as collateral	$ 85,000,000	$ 85,000,000
Credit Agreement
Short-term Debt [Line Items]
Number of bank counterparties	17	17

Carrying value as of the balance sheet date of [accrued] interest payable on all forms of debt, including trade payables, that has been incurred and is unpaid. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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