Consolidated Statments of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Activities:
Net income	$ 819,122	$ 839,519	$ 582,237
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization expense	166,812	161,741	142,230
Loss on sale and retirement of assets	7,871	7,223	7,835
Earnings of non-controlled entities	(66,483)	(19,394)	(6,275)
Distributions from investments in non-controlled entities	66,285	3,086	2,494
Equity-based incentive compensation expense	24,245	27,284	24,083
Settlement cost, amortization of prior service credit and actuarial loss	3,478	2,115	1,964
Changes in components of operating assets and liabilities (Note 3)	48,362	85,727	18,508
Net cash provided by operating activities	1,069,692	1,107,301	773,076
Investing Activities:
Additions to property, plant and equipment	(621,151)	(363,250)	(421,435)
Proceeds from sale and disposition of assets	3,371	10,780	3,610
Acquisition of assets	0	(75,000)	(22,506)
Acquisition of business	(54,678)	0	(192,000)
Investments in non-controlled entities	(152,466)	(408,001)	(250,495)
Distributions in excess of earnings of non-controlled entities	14,155	5,487	780
Net cash used by investing activities	(810,769)	(829,984)	(882,046)
Financing Activities:
Distributions paid	(662,948)	(568,806)	(475,461)
Net commercial paper borrowings (repayments)	(16,981)	296,942	0
Borrowings under long-term notes	499,589	257,713	298,680
Payments on notes	0	(250,000)	0
Debt placement costs	(6,223)	(2,912)	(4,849)
Net payment on financial derivatives	(42,908)	(3,613)	(184)
Settlement of tax withholdings on equity-based incentive compensation	(17,784)	(14,813)	(12,259)
Net cash used by financing activities	(247,255)	(285,489)	(194,073)
Change in cash and cash equivalents	11,668	(8,172)	(303,043)
Cash and cash equivalents at beginning of period	17,063	25,235	328,278
Cash and cash equivalents at end of period	28,731	17,063	25,235
Noncash Investing and Financing Items [Abstract]
Contribution of property, plant and equipment to a non-controlled entity	13,252	0	0
Issuance of limited partner units in settlement of equity-based incentive plan awards	8,045	7,315	6,404
(1) Additions to property, plant and equipment	(623,289)	(366,445)	(383,757)
Increase (decrease) in accounts payable related to capital expenditures	2,138	3,195	(37,678)
Additions to property, plant and equipment	$ (621,151)	$ (363,250)	$ (421,435)