Compensation Plans (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Feb. 11, 2011	Jan. 25, 2011	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Dec. 31, 2009
Shares available for grant			2,200,000		2,200,000
Shares available for grant units for which vesting requirements have been satisfied		91,100
Shares forfeiture, net		2,500
Common units issued, vested	58,886
Common unit authorized, grants		110,000
Granted shares from authorized					108,416
Intrinsic value of granted shares					$ 66.84
Share based compensation expense			$ 1.4	$ 1.0	$ 2.5	$ 1.9
Future forfeitures					0		0	0
Unrecognized compensation expense			9.2		9.2
Expense is expected to be recognized over a weighted-average period, years					1.4
Intrinsic value of the non-vested LTIP, grants					29.9
Total obligation associated with the LTIP					6.8
Reclassification of SERP and Directors Compensation to Partners' Capital			9.2		9.2
Reclassification date						January 1, 2011
SERP And Directors Deferred Compensation Plan [Member]
Total obligation associated with the LTIP					9.6
Phantom Units [Member]
Common units issued, vested					5,898	6,283
Weighted average grant date fair value					$ 71.96	$ 44.97
Deferred compensation plan expense			0.2	0.6	0.4	0.6
Common units outstanding			146,143		146,143
Intrinsic value of vested units					$ 11.3