Debt and Credit Agreements - Schedule of Notes Issued (Details) - Senior Notes - USD ($)	1 Months Ended
	Sep. 30, 2014	Dec. 31, 2010	Jul. 31, 2008
6.51% weighted-average senior notes
Debt Instrument [Line Items]
Principal			$ 425,000,000
6.51% weighted-average senior notes | Tranche 1
Debt Instrument [Line Items]
Principal			$ 245,000,000
Term			10 years
Coupon (as a percent)			6.44%
6.51% weighted-average senior notes | Tranche 2
Debt Instrument [Line Items]
Principal			$ 100,000,000
Term			12 years
Coupon (as a percent)			6.54%
6.51% weighted-average senior notes | Tranche 3
Debt Instrument [Line Items]
Principal			$ 80,000,000
Term			15 years
Coupon (as a percent)			6.69%
5.58% weighted-average senior notes
Debt Instrument [Line Items]
Principal		$ 175,000,000
5.58% weighted-average senior notes | Tranche 1
Debt Instrument [Line Items]
Principal		$ 88,000,000
Term		10 years
Coupon (as a percent)		5.42%
5.58% weighted-average senior notes | Tranche 2
Debt Instrument [Line Items]
Principal		$ 25,000,000
Term		12 years
Coupon (as a percent)		5.59%
5.58% weighted-average senior notes | Tranche 3
Debt Instrument [Line Items]
Principal		$ 62,000,000
Term		15 years
Coupon (as a percent)		5.80%
3.65% weighted-average senior notes
Debt Instrument [Line Items]
Principal	$ 925,000,000
3.65% weighted-average senior notes | Tranche 1
Debt Instrument [Line Items]
Principal	$ 100,000,000
Term	7 years
Coupon (as a percent)	3.24%
3.65% weighted-average senior notes | Tranche 2
Debt Instrument [Line Items]
Principal	$ 575,000,000
Term	10 years
Coupon (as a percent)	3.67%
3.65% weighted-average senior notes | Tranche 3
Debt Instrument [Line Items]
Principal	$ 250,000,000
Term	12 years
Coupon (as a percent)	3.77%