Debt (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Short-term Debt [Abstract]
Short-term Debt	$ 270,035	$ 234,700
Commercial Paper	220,000	234,700
Bank Loans	$ 50,000	$ 0
Short-term Debt, Weighted Average Interest Rate	0.60%	0.40%
Commercial Paper, Maximum Maturity	77 days
Commercial Paper, Average Maturity	36 days
Maturities of Long-term Debt [Abstract]
Long-term Debt, Current Maturities	$ 25,000	$ 40,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	40,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	22,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	30,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	75,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	409,700
Debt Outstanding
Secured Debt	601,700	641,700
Long-term Debt	601,700	661,700
Long-term Debt, Current Maturities	25,000	40,000
Long-term debt, noncurrent	576,700	621,700
Liabilities, Fair Value Disclosure [Abstract]
Debt Instrument, Fair Value Disclosure	667,168	756,808
Note 4700 Series B Due 2015 [Member]
Debt Outstanding
Secured Debt	$ 0	$ 40,000
Debt Instrument, Interest Rate, Stated Percentage	4.70%	4.70%
Repayments of Long-term Debt [Abstract]
Extinguishment of Debt, Amount	$ 40,000
Note 5150 Series B Due 2016 [Member]
Debt Outstanding
Secured Debt	$ 25,000	$ 25,000
Debt Instrument, Interest Rate, Stated Percentage	5.15%	5.15%
Note 7000 Series B Due 2017 [Member]
Debt Outstanding
Secured Debt	$ 40,000	$ 40,000
Debt Instrument, Interest Rate, Stated Percentage	7.00%	7.00%
Note 6600 Series B Due 2018 [Member]
Debt Outstanding
Secured Debt	$ 22,000	$ 22,000
Debt Instrument, Interest Rate, Stated Percentage	6.60%	6.60%
Note 8310 Series B Due 2019 [Member]
Debt Outstanding
Secured Debt	$ 10,000	$ 10,000
Debt Instrument, Interest Rate, Stated Percentage	8.31%	8.31%
Note 7630 Series B Due 2019 [Member]
Debt Outstanding
Secured Debt	$ 20,000	$ 20,000
Debt Instrument, Interest Rate, Stated Percentage	7.63%	7.63%
Note 5370 Series B Due 2020 [Member]
Debt Outstanding
Secured Debt	$ 75,000	$ 75,000
Debt Instrument, Interest Rate, Stated Percentage	5.37%	5.37%
Note 9050 Series A Due 2021 [Member]
Debt Outstanding
Secured Debt	$ 10,000	$ 10,000
Debt Instrument, Interest Rate, Stated Percentage	9.05%	9.05%
Note 3176 Series B Due 2021 [Member]
Debt Outstanding
Secured Debt	$ 50,000	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	3.176%	3.176%
Note 3542 Series B Due 2023 [Member]
Debt Outstanding
Secured Debt	$ 50,000	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	3.542%	3.542%
Note 5620 Series B Due 2023 [Member]
Debt Outstanding
Secured Debt	$ 40,000	$ 40,000
Debt Instrument, Interest Rate, Stated Percentage	5.62%	5.62%
Note 7720 Series B Due 2025 [Member]
Debt Outstanding
Secured Debt	$ 20,000	$ 20,000
Debt Instrument, Interest Rate, Stated Percentage	7.72%	7.72%
Note 6520 Series B Due 2025 [Member]
Debt Outstanding
Secured Debt	$ 10,000	$ 10,000
Debt Instrument, Interest Rate, Stated Percentage	6.52%	6.52%
Note 7050 Series B Due 2026 [Member]
Debt Outstanding
Secured Debt	$ 20,000	$ 20,000
Debt Instrument, Interest Rate, Stated Percentage	7.05%	7.05%
Note 7000 Series B Due 2027 [Member]
Debt Outstanding
Secured Debt	$ 20,000	$ 20,000
Debt Instrument, Interest Rate, Stated Percentage	7.00%	7.00%
Note 6650 Series B Due 2027 [Member]
Debt Outstanding
Secured Debt	$ 19,700	$ 19,700
Debt Instrument, Interest Rate, Stated Percentage	6.65%	6.65%
Note 6650 Series B Due 2028 [Member]
Debt Outstanding
Secured Debt	$ 10,000	$ 10,000
Debt Instrument, Interest Rate, Stated Percentage	6.65%	6.65%
Note 7740 Series B Due 2030 [Member]
Debt Outstanding
Secured Debt	$ 20,000	$ 20,000
Debt Instrument, Interest Rate, Stated Percentage	7.74%	7.74%
Note 7850 Series B Due 2030 [Member]
Debt Outstanding
Secured Debt	$ 10,000	$ 10,000
Debt Instrument, Interest Rate, Stated Percentage	7.85%	7.85%
Note 5820 Series B Due 2032 [Member]
Debt Outstanding
Secured Debt	$ 30,000	$ 30,000
Debt Instrument, Interest Rate, Stated Percentage	5.82%	5.82%
Note 5660 Series B Due 2033 [Member]
Debt Outstanding
Secured Debt	$ 40,000	$ 40,000
Debt Instrument, Interest Rate, Stated Percentage	5.66%	5.66%
Note 5250 Series B Due 2035 [Member]
Debt Outstanding
Secured Debt	$ 10,000	$ 10,000
Debt Instrument, Interest Rate, Stated Percentage	5.25%	5.25%
Note 4000 Due 2042 [Member]
Debt Outstanding
Secured Debt	$ 50,000	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	4.00%	4.00%
Secured Notes Due 2016 [Member]
Debt Outstanding
Subsidiary Senior Secured Notes	$ 0	$ 20,000
Debt Instrument, Interest Rate, Stated Percentage	7.75%
Repayments of Long-term Debt [Abstract]
Extinguishment of Debt, Amount	$ 20,000
Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	300,000
Line of Credit Facility Contingent Increase To Maximum Borrowing Capacity	$ 450,000
Line of Credit Facility, Expiration Date	Dec. 20, 2019
Line of Credit Facility, Amount Outstanding	$ 0	0
Debt Instrument Covenant Consolidated Indebtedness To Capitalization Ratio	70.00%
Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 100,000
Letters of Credit Outstanding, Amount	$ 0	$ 0

Commercial Paper, Average Maturity (duration)

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Commercial Paper, Maximum Maturity (duration)

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Represents the maximum ratio of consolidated indebtedness to total capitalizationwhich is necessary to be maintained under the terms of the line of credit facility.

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Increased maximum borrowing capacity under the credit facility which the reporting entity may request.

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Amounts due on short-term bank loans secured by customers' securities on the balance sheet date.

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Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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

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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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Gross amount of debt extinguished.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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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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Date the credit facility terminates, in CCYY-MM-DD format.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

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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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Weighted average interest rate of short-term debt outstanding.

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