Business Segment Information (Reconciliation Of Segment Margin To (Loss) Income Before Income Taxes) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Segment Reporting [Abstract]
Segment margin	$ 62,831	[1]	$ 47,673	[1]	$ 123,083	[1]	$ 96,828	[1]
Corporate general and administrative expenses	(8,707)		(7,689)		(17,328)		(15,073)
Depreciation and amortization	(15,357)		(14,253)		(30,392)		(28,156)
Net loss on disposal of surplus assets	(473)		(249)		(217)		(238)
Interest expense	(10,228)		(9,011)		(20,824)		(17,710)
Distributable cash from equity investees in excess of equity in earnings	(6,752)		(4,921)		(13,485)		(8,224)
Non-cash items not included in segment margin	(1,577)		7,103		(253)		(331)
Cash payments from direct financing leases in excess of earnings	(1,249)		(1,141)		(2,470)		(2,254)
Income before income taxes	$ 18,488		$ 17,512		$ 38,114		$ 24,842

[1] A reconciliation of Segment Margin to income before income taxes for the periods presented is as follows: Three Months EndedJune 30, Six Months EndedJune 30, 2012 2011 2012 2011Segment Margin$62,831 $47,673 $123,083 $96,828Corporate general and administrative expenses(8,707) (7,689) (17,328) (15,073)Depreciation and amortization(15,357) (14,253) (30,392) (28,156)Net loss on disposal of surplus assets(473) (249) (217) (238)Interest expense(10,228) (9,011) (20,824) (17,710)Distributable cash from equity investees in excess of equity in earnings(6,752) (4,921) (13,485) (8,224)Non-cash items not included in segment margin(1,577) 7,103 (253) (331)Cash payments from direct financing leases in excess of earnings(1,249) (1,141) (2,470) (2,254)Income before income taxes$18,488 $17,512 $38,114 $24,842