Consolidated Statement of Changes in Partner Capital Statement - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Adjustment due to change inaccounting policy (note 1)				$ 30,903
Net income	$ 52,690	$ 48,589	$ 144,183	102,054
Other comprehensive income	(8,520)	5,748	(14,048)	17,272
Contributions from partners	1,001		1,896
Distributions to partners	(1,299)	(1,141)	(2,864)	(2,159)
Reallocation of limited partner's interest			0
Restricted units issued as a result of amortization of restricted stock issued by Parent Company, net of amortization	3,952	4,014	7,904	8,135
Partners Capital Total [Member]
Adjustment due to change inaccounting policy (note 1)				30,901
Net income	51,728	47,841	142,174	100,500
Other comprehensive income	(7,990)	5,553	(13,159)	16,594
Reallocation of limited partner's interest			66
Restricted units issued as a result of amortization of restricted stock issued by Parent Company, net of amortization	3,952	4,014	7,904	8,135
Partnership Interest [Member]
Beginning Balance	6,402,773	6,605,381	6,450,168	6,733,054
Adjustment due to change inaccounting policy (note 1)				30,903
Adjusted balance				6,763,957
Net income	52,690	48,589	144,183	102,054
Other comprehensive income	(8,520)	5,748	(14,048)	17,272
Deferred compensation plan, net		(1)		(4)
Contributions from partners	1,001		1,896
Distributions to partners	(99,499)	(95,366)	(199,260)	(191,785)
Reallocation of limited partner's interest			0
Restricted units issued as a result of amortization of restricted stock issued by Parent Company, net of amortization	3,952	4,014	7,904	8,135
Common units repurchased and retired as a result of common stock repurchased and retired by Parent Company			(32,778)	(124,989)
Common units issued as a result of common stock issued by Parent Company, net of repurchases		423		(5,852)
Common units redeemed as a result of common stock redeemed by Parent Company, net of issuances	451		(5,217)
Ending Balance	6,352,848	6,568,788	6,352,848	6,568,788
Partnership Interest [Member] | Partners Capital Total [Member]
Beginning Balance	6,361,336	6,575,148	6,408,636	6,702,959
Adjustment due to change inaccounting policy (note 1)				30,901
Adjusted balance				6,733,860
Net income	51,837	47,941	142,473	100,712
Other comprehensive income	(8,007)	5,565	(13,187)	16,629
Deferred compensation plan, net		(1)		(4)
Contributions from partners	0		0
Distributions to partners	(98,200)	(94,225)	(196,396)	(189,626)
Reallocation of limited partner's interest			(66)
Restricted units issued as a result of amortization of restricted stock issued by Parent Company, net of amortization	3,952	4,014	7,904	8,135
Common units repurchased and retired as a result of common stock repurchased and retired by Parent Company			(32,778)	(124,989)
Common units issued as a result of common stock issued by Parent Company, net of repurchases		423		(5,852)
Common units redeemed as a result of common stock redeemed by Parent Company, net of issuances	451		(5,217)
Ending Balance	6,311,369	6,538,865	6,311,369	6,538,865
Partnership Interest [Member] | AOCI Attributable to Parent [Member]
Beginning Balance	(6,096)	4,764	(927)	(6,289)
Adjustment due to change inaccounting policy (note 1)				12
Adjusted balance				(6,277)
Net income	0	0	0	0
Other comprehensive income	(7,990)	5,553	(13,159)	16,594
Deferred compensation plan, net		0		0
Contributions from partners	0		0
Distributions to partners	0	0	0	0
Reallocation of limited partner's interest			0
Restricted units issued as a result of amortization of restricted stock issued by Parent Company, net of amortization	0	0	0	0
Common units repurchased and retired as a result of common stock repurchased and retired by Parent Company			0	0
Common units issued as a result of common stock issued by Parent Company, net of repurchases		0		0
Common units redeemed as a result of common stock redeemed by Parent Company, net of issuances	0		0
Ending Balance	(14,086)	10,317	(14,086)	10,317
Partnership Interest [Member] | Noncontrolling Interest [Member]
Beginning Balance	41,437	30,233	41,532	30,095
Adjustment due to change inaccounting policy (note 1)				2
Adjusted balance				30,097
Net income	853	648	1,710	1,342
Other comprehensive income	(513)	183	(861)	643
Deferred compensation plan, net		0		0
Contributions from partners	1,001		1,896
Distributions to partners	(1,299)	(1,141)	(2,864)	(2,159)
Reallocation of limited partner's interest			66
Restricted units issued as a result of amortization of restricted stock issued by Parent Company, net of amortization	0	0	0	0
Common units repurchased and retired as a result of common stock repurchased and retired by Parent Company			0	0
Common units issued as a result of common stock issued by Parent Company, net of repurchases		0		0
Common units redeemed as a result of common stock redeemed by Parent Company, net of issuances	0		0
Ending Balance	41,479	29,923	41,479	29,923
Partnership Interest [Member] | General Partner [Member]
Beginning Balance	6,356,791	6,559,537	6,398,897	6,698,341
Adjustment due to change inaccounting policy (note 1)				30,889
Adjusted balance				6,729,230
Net income	51,728	47,841	142,174	100,500
Other comprehensive income	0	0	0	0
Deferred compensation plan, net		(1)		(4)
Contributions from partners	0		0
Distributions to partners	(97,995)	(94,031)	(195,987)	(189,237)
Reallocation of limited partner's interest			(66)
Restricted units issued as a result of amortization of restricted stock issued by Parent Company, net of amortization	3,952	4,014	7,904	8,135
Common units repurchased and retired as a result of common stock repurchased and retired by Parent Company			(32,778)	(124,989)
Common units issued as a result of common stock issued by Parent Company, net of repurchases		423		(5,852)
Common units redeemed as a result of common stock redeemed by Parent Company, net of issuances	451		(5,217)
Ending Balance	6,314,927	6,517,783	6,314,927	6,517,783
Partnership Interest [Member] | Limited Partner [Member]
Beginning Balance	10,641	10,847	10,666	10,907
Adjustment due to change inaccounting policy (note 1)				0
Adjusted balance				10,907
Net income	109	100	299	212
Other comprehensive income	(17)	12	(28)	35
Deferred compensation plan, net		0		0
Contributions from partners	0		0
Distributions to partners	(205)	(194)	(409)	(389)
Reallocation of limited partner's interest			0
Restricted units issued as a result of amortization of restricted stock issued by Parent Company, net of amortization	0	0	0	0
Common units repurchased and retired as a result of common stock repurchased and retired by Parent Company			0	0
Common units issued as a result of common stock issued by Parent Company, net of repurchases		0		0
Common units redeemed as a result of common stock redeemed by Parent Company, net of issuances	0		0
Ending Balance	$ 10,528	$ 10,765	$ 10,528	$ 10,765