Cash Distributions Paid to Unitholders (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010		Dec. 31, 2009
Distribution Made to Member or Limited Partner [Line Items]
Limited Partners	$ 341,369	$ 197,683		$ 183,753
General Partner		51,901	[1]	46,623	[1]
Total Cash Distributions	341,369	249,584		230,376
Period 1
Distribution Made to Member or Limited Partner [Line Items]
Record Date	Feb. 21, 2011	Feb. 16, 2010		Feb. 12, 2009
Payment Date	Feb. 28, 2011	Feb. 26, 2010		Feb. 27, 2009
Amount Per LP Unit	$ 0.9875	$ 0.9375		$ 0.8875
Limited Partners	79,603	48,425		42,930
General Partner		12,543	[1]	10,721	[1]
Total Cash Distributions	79,603	60,968		53,651
Period 2
Distribution Made to Member or Limited Partner [Line Items]
Record Date	May 16, 2011	May 17, 2010		May 11, 2009
Payment Date	May 31, 2011	May 28, 2010		May 29, 2009
Amount Per LP Unit	$ 1.0000	$ 0.9500		$ 0.9000
Limited Partners	86,153	49,048		46,227
General Partner		12,835	[1]	11,686	[1]
Total Cash Distributions	86,153	61,883		57,913
Period 3
Distribution Made to Member or Limited Partner [Line Items]
Record Date	Aug. 15, 2011	Aug. 16, 2010		Aug. 07, 2009
Payment Date	Aug. 31, 2011	Aug. 31, 2010		Aug. 31, 2009
Amount Per LP Unit	$ 1.0125	$ 0.9625		$ 0.9125
Limited Partners	87,236	49,778		46,877
General Partner		13,121	[1]	11,965	[1]
Total Cash Distributions	87,236	62,899		58,842
Period 4
Distribution Made to Member or Limited Partner [Line Items]
Record Date	Nov. 14, 2011	Nov. 15, 2010		Nov. 12, 2009
Payment Date	Nov. 30, 2011	Nov. 30, 2010		Nov. 30, 2009
Amount Per LP Unit	$ 1.0250	$ 0.9750		$ 0.9250
Limited Partners	88,377	50,432		47,719
General Partner		13,402	[1]	12,251	[1]
Total Cash Distributions	88,377	63,834		59,970
General Partner
Distribution Made to Member or Limited Partner [Line Items]
Total Cash Distributions		49,808		40,752
General Partner | Period 1
Distribution Made to Member or Limited Partner [Line Items]
Record Date		Feb. 16, 2010		Feb. 12, 2009
Payment Date		Feb. 26, 2010		Feb. 27, 2009
Amount Per Unit		$ 0.410		$ 0.330
Total Cash Distributions		11,603		9,339
General Partner | Period 2
Distribution Made to Member or Limited Partner [Line Items]
Record Date		May 17, 2010		May 11, 2009
Payment Date		May 28, 2010		May 29, 2009
Amount Per Unit		$ 0.430		$ 0.350
Total Cash Distributions		12,169		9,905
General Partner | Period 3
Distribution Made to Member or Limited Partner [Line Items]
Record Date		Aug. 16, 2010		Aug. 07, 2009
Payment Date		Aug. 31, 2010		Aug. 31, 2009
Amount Per Unit		$ 0.450		$ 0.370
Total Cash Distributions		12,735		10,472
General Partner | Period 4
Distribution Made to Member or Limited Partner [Line Items]
Record Date		Nov. 15, 2010		Nov. 12, 2009
Payment Date		Nov. 30, 2010		Nov. 30, 2009
Amount Per Unit		$ 0.470		$ 0.390
Total Cash Distributions		$ 13,301		$ 11,036

[1] Includes amounts paid to our general partner for its incentive distribution rights.