Subsequent Events (Details) - USD ($)											9 Months Ended		12 Months Ended
	Nov. 13, 2015	[1]	Oct. 31, 2015	Aug. 14, 2015	May. 15, 2015	Feb. 13, 2015	Nov. 14, 2014	Aug. 14, 2014	May. 15, 2014	Feb. 14, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2015	Dec. 31, 2014
Subsequent Event [Line Items]
Cash distribution per unit				$ 0.7400	$ 0.7175	$ 0.6950	$ 0.6675	$ 0.64	$ 0.6125	$ 0.585	$ 2.1525	$ 1.8375		$ 2.505
Total cash distributions				$ 168,296,000	$ 163,178,000	$ 158,061,000	$ 151,568,000	$ 145,324,000	$ 139,079,000	$ 132,835,000	$ 489,535,000	$ 417,238,000		$ 568,806,000
Debt placement costs											4,754,000	$ 2,912,000
Revolving Credit Facility [Member] | Senior Notes [Member]
Subsequent Event [Line Items]
Borrowing capacity											$ 1,000,000,000.0
Unused commitment fee											0.125%
Revolving Credit Facility [Member] | Senior Notes [Member] | Minimum [Member]
Subsequent Event [Line Items]
Debt Instrument, Basis Spread on Variable Rate											1.00%
Unused commitment fee											0.10%
Revolving Credit Facility [Member] | Senior Notes [Member] | Maximum [Member]
Subsequent Event [Line Items]
Debt Instrument, Basis Spread on Variable Rate											1.75%
Unused commitment fee											0.28%
Subsequent Event [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Subsequent Event [Line Items]
Debt Instrument, Basis Spread on Variable Rate			1.00%
Subsequent Event [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Subsequent Event [Line Items]
Debt Instrument, Basis Spread on Variable Rate			1.625%
Subsequent Event [Member] | Revolving Credit Facility [Member] | Notes Payable to Banks [Member]
Subsequent Event [Line Items]
Borrowing capacity			$ 250,000,000
Debt Instrument, Term			364 days
Unused commitment fee			0.10%
Debt placement costs			$ 100,000.0
Subsequent Event [Member] | Revolving Credit Facility [Member] | Notes Payable to Banks [Member] | Minimum [Member]
Subsequent Event [Line Items]
Unused commitment fee			0.08%
Subsequent Event [Member] | Revolving Credit Facility [Member] | Notes Payable to Banks [Member] | Maximum [Member]
Subsequent Event [Line Items]
Unused commitment fee			0.225%
Subsequent Event [Member] | Revolving Credit Facility [Member] | Senior Notes [Member]
Subsequent Event [Line Items]
Borrowing capacity			$ 1,000,000,000.0
Unused commitment fee			0.125%
Debt placement costs			$ 1,400,000.0
Subsequent Event [Member] | Revolving Credit Facility [Member] | Senior Notes [Member] | Minimum [Member]
Subsequent Event [Line Items]
Unused commitment fee			0.10%
Subsequent Event [Member] | Revolving Credit Facility [Member] | Senior Notes [Member] | Maximum [Member]
Subsequent Event [Line Items]
Unused commitment fee			0.275%
Scenario, Forecast
Subsequent Event [Line Items]
Cash distribution per unit	$ 0.7625												$ 2.915
Total cash distributions	$ 173,413,000												$ 662,948,000

[1] Our general partner's board of directors declared this cash distribution on October 22, 2015 to be paid on November 13, 2015 to unitholders of record at the close of business on November 2, 2015.