Consolidated Statement of Changes in Partnership Capital Statement - USD ($) $ in Thousands	Total	Partners Capital Total [Member]	AOCI Attributable to Parent [Member]	Noncontrolling Interest [Member]	General Partner [Member]	Limited Partner [Member]	Partnership Interest [Member]	Partners Capital Total [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,861,134	$ 1,841,928	$ (17,404)	$ 19,206	$ 1,860,758	$ (1,426)
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	12,161		0	0	12,161	0		$ 12,161
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	188,847	187,709	0	1,138	187,390	319	$ 188,847
Current period other comprehensive income, net	(40,615)	(40,414)	(40,344)		0	(70)	(40,615)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest							(201)
Partners' Capital Account, Contributions	16,204	0	0	16,204	0	0
Distributions To Partners	180,031	175,488	0	4,543	175,188	300
Partners' Capital Account, Redemptions	300	300	0	0	0	300
Dividends, Preferred Stock, Cash	21,062		0	0	(21,062)	0		(21,062)
Common units exchanged for common stock of the parent company	0		0	0	137	(137)		0
Common unit issued as a result of common stock issued by Parent Company, net of purchases	100,144		0	0	100,144	0		100,144
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,936,482	1,904,678	(57,748)	31,804	1,964,340	(1,914)
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	13,869		0	0	13,869	0		13,869
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	152,543	150,296	0	2,247	150,056	240	152,543
Current period other comprehensive income, net	(980)	(947)	(945)		0	(2)	(980)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest							(33)
Partners' Capital Account, Contributions	717	0	0	717	0	0
Distributions To Partners	188,987	184,738	0	4,249	184,439	299
Dividends, Preferred Stock, Cash	21,062		0	0	(21,062)	0		(21,062)
Common unit issued as a result of common stock issued by Parent Company, net of purchases	190,038		0	0	190,038	0		190,038
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,082,620	2,052,134	(58,693)	30,486	2,112,802	(1,975)	2,082,620
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	13,421		0	0	13,421	0		13,421
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	166,992	165,179	0	1,813	164,922	257	166,992
Current period other comprehensive income, net	40,831	40,405	40,347		0	58	40,831
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest							426
Partners' Capital Account, Contributions	8,760	0	0	8,760	0	0
Distributions To Partners	209,261	202,406	0	6,855	202,099	307
Noncontrolling Interest, Decrease from Deconsolidation	0	(538)	0	538	(538)	0
Partners' Capital Account, Redemptions	0	0	0	0	0	0
Dividends, Preferred Stock, Cash	21,062		0	0	(21,062)	0		(21,062)
Common unit issued as a result of common stock issued by Parent Company, net of purchases	542,201		0	0	542,201	0		$ 542,201
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 2,624,502	$ 2,589,334	$ (18,346)	$ 35,168	$ 2,609,647	$ (1,967)	$ 2,624,502