Partnership Capital Subject to Mandatory Redemption (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Partner capital issued:
Limited partnership capital	$ 650,735	$ 662,226
Subordinated limited partnership capital	283,709	255,414
General partnership capital issued	1,048,067	927,578
Total partner capital issued	1,982,511	1,845,218
Partnership loans outstanding:
General partnership loans outstanding at beginning of period	(86,853)	0
General partnership loans issued during the period	(94,170)	(91,693)
Repayment of general partnership loans during the period	10,759	4,840
Total partnership loans outstanding	(170,264)	(86,853)
Partnership capital subject to mandatory redemption, net of reserve for anticipated withdrawals	1,812,247	1,758,365	1,496,725	1,436,528
Reserve for anticipated withdrawals	170,646	147,412	108,248	30,284
Partnership capital subject to mandatory redemption	$ 1,982,893	$ 1,905,777	$ 1,604,973	$ 1,466,812