Long-Term Debt (Details of the Company's Credit Agreement) (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Amount drawn		$ 3,809,600	$ 3,926,321
Revolving Credit Facility [Member] | Credit Agreement [Member]
Line of Credit Facility [Line Items]
Available		$ 1,220,107	$ 1,149,813
Commitment - rate applicable		0.12%	0.15%
Amount drawn	[1]	$ 195,166	$ 192,101
Letter of Credit [Member] | Credit Agreement [Member]
Line of Credit Facility [Line Items]
Letter of credit		147,228	220,586
LIBOR [Member] | Term Loan Facility [Member] | Credit Agreement [Member]
Line of Credit Facility [Line Items]
Amount drawn		$ 1,637,500	$ 1,637,500
Interest rate applicable		3.19%	2.77%
Canadian Prime Rate [Member] | Revolving Credit Facility [Member] | Credit Agreement [Member]
Line of Credit Facility [Line Items]
Amount drawn		$ 9,113	$ 16,739
Interest rate applicable		3.58%	3.45%
Canadian Bankers Acceptance Loan [Member] | Revolving Credit Facility [Member] | Credit Agreement [Member]
Line of Credit Facility [Line Items]
Amount drawn		$ 186,053	$ 175,362
Interest rate applicable		2.77%	2.64%

[1] Interest rates represent the interest rates incurred at June 30, 2018