Consolidated Statement of Changes in Partner Capital Statement - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Partnership Interest [Member]
Common unit redeemed as result of common stock redeemed by parent	$ (6,275)
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			$ 30,903
Partners' Capital, Adjusted Balance			6,763,957
Net income	53,465
Other comprehensive income (loss)	11,524
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	460
Deferred Compensation Plan	(3)
Partners' Capital Account, Distributions	(96,419)
Common units repurchased and retired as a result of common stock repurchased and retired by Parent Company	(124,989)
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	4,121
Partners' Capital, Including Portion Attributable to Noncontrolling Interest	6,605,381	$ 6,450,168	6,733,054
Partnership Interest [Member] | Noncontrolling Interest [Member]
Common unit redeemed as result of common stock redeemed by parent	0
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			2
Partners' Capital, Adjusted Balance			30,097
Net income	694
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	460
Deferred Compensation Plan	0
Partners' Capital Account, Distributions	(1,018)
Common units repurchased and retired as a result of common stock repurchased and retired by Parent Company	0
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	0
Partners' Capital, Including Portion Attributable to Noncontrolling Interest	30,233	41,532	30,095
Partnership Interest [Member] | Partners Capital Total [Member]
Common unit redeemed as result of common stock redeemed by parent	(6,275)
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			30,901
Partners' Capital, Adjusted Balance			6,733,860
Net income	52,771
Other comprehensive income (loss)	11,064
Deferred Compensation Plan	(3)
Partners' Capital Account, Distributions	(95,401)
Common units repurchased and retired as a result of common stock repurchased and retired by Parent Company	(124,989)
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	4,121
Partners' Capital, Including Portion Attributable to Noncontrolling Interest	6,575,148	6,408,636	6,702,959
Partnership Interest [Member] | AOCI Attributable to Parent [Member]
Common unit redeemed as result of common stock redeemed by parent	0
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			12
Partners' Capital, Adjusted Balance			(6,277)
Net income	0
Other comprehensive income (loss)	11,041
Deferred Compensation Plan	0
Partners' Capital Account, Distributions	0
Common units repurchased and retired as a result of common stock repurchased and retired by Parent Company	0
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	0
Partners' Capital, Including Portion Attributable to Noncontrolling Interest	4,764	(927)	(6,289)
Partnership Interest [Member] | Limited Partner [Member]
Common unit redeemed as result of common stock redeemed by parent	0
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			0
Partners' Capital, Adjusted Balance			10,907
Net income	111
Other comprehensive income (loss)	23
Deferred Compensation Plan	0
Partners' Capital Account, Distributions	(194)
Common units repurchased and retired as a result of common stock repurchased and retired by Parent Company	0
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	0
Partners' Capital, Including Portion Attributable to Noncontrolling Interest	10,847	10,666	10,907
Partnership Interest [Member] | General Partner [Member]
Common unit redeemed as result of common stock redeemed by parent	(6,275)
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			30,889
Partners' Capital, Adjusted Balance			6,729,230
Net income	52,660
Other comprehensive income (loss)	0
Deferred Compensation Plan	(3)
Partners' Capital Account, Distributions	(95,207)
Common units repurchased and retired as a result of common stock repurchased and retired by Parent Company	(124,989)
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	4,121
Partners' Capital, Including Portion Attributable to Noncontrolling Interest	6,559,537	$ 6,398,897	6,698,341
AOCI Attributable to Parent [Member]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			$ 12
Other comprehensive income (loss)	$ 11,041