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Debt (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2013	Nov. 24, 2013
Line of Credit Facility [Line Items]
Interest rate description	Borrowings under the 2013 Credit Agreement bear interest at (1) a rate which is the prime commercial lending rate announced by Bank of Montreal from time to time (3.50% at March 31, 2016) plus 0.25% to 0.75%, based on certain financial tests or (2) United States dollar LIBOR (0.43% at March 31, 2016) plus 1.25% to 1.75%, based on certain financial tests. The interest rate in effect at March 31, 2016 was 1.68%. Fees for letters of credit issued under the 2013 Revolving Credit Facility range from 1.25% to 1.75% of the respective face amounts of outstanding letters of credit and are computed based on certain financial tests.
Letters of credit outstanding (in US dollars)	$ 103,900,000	$ 99,000,000
2011 Credit Agreement
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity (in US dollars)				$ 750,000,000
2013 Credit Agreement
Line of Credit Facility [Line Items]
Expiration date of revolving credit facility	Nov. 25, 2018
Interest rate	1.68%
Debt issuance costs, capitalized (in US dollars)			$ 3,000,000
2013 Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity (in US dollars)			750,000,000
Increase in borrowing capacity (in US dollars)		1,050,000,000
Letters of credit maximum borrowing capacity (in US dollars)		250,000,000
Commitment fee percentage of unused amount	0.20%
Borrowings under revolving credit facility (in US dollars)	$ 0	0
2013 Term Loan
Line of Credit Facility [Line Items]
Term Loan (in US dollars)	310,625,000	$ 315,000,000	$ 350,000,000
Periodic principal payment on term loan (in US dollars)	$ 4,400,000
Minimum [Member] | 2013 Revolving Credit Facility
Line of Credit Facility [Line Items]
Commitment fee percentage of unused amount	0.20%
Letter of credit fees	1.25%
Maximum [Member] | 2013 Revolving Credit Facility
Line of Credit Facility [Line Items]
Commitment fee percentage of unused amount	0.30%
Letter of credit fees	1.75%
Prime Rate [Member] | 2013 Credit Agreement
Line of Credit Facility [Line Items]
Commercial lending rate	3.50%
Prime Rate [Member] | Minimum [Member] | 2013 Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.25%
Prime Rate [Member] | Maximum [Member] | 2013 Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.75%
London Interbank Offered Rate (LIBOR) [Member] | 2013 Credit Agreement
Line of Credit Facility [Line Items]
Commercial lending rate	0.43%
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | 2013 Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.25%
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] | 2013 Credit Agreement
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.75%

X

- Definition

The amount as of the balance sheet date of capitalized costs associated with the issuance of debt instruments that will be charged against earnings over the life of the debt instruments to which such costs pertain. Such amount is before the consideration of accumulated amortization.

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Amount of increase in borrowing capacity if additional lenders are identified and/or existing lenders are willing to increase their current commitments.

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Letters of credit maximum borrowing capacity.

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Range of letter of credit fees.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Amount of the required periodic payments applied to principal.

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

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The effective interest rate at the end of the reporting period.

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Description of the interest rate for the amounts borrowed under the credit facility, including the terms and the method for determining the interest rate (for example, fixed or variable, LIBOR plus a percentage, increasing rate, timing of interest rate resets, remarketing provisions).

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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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The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit 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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The interest rate applicable to the portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a rate subject to change from time to time.

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The carrying value as of the balance sheet date of the noncurrent portion 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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