Accounting Policies (Comparison of Reported Condensed Consolidated Statement of Cash Flows to the Pro-forma Amounts had the Previous U.S. GAAP Guidance been in Effect) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Cash Flows from Operating Activities:
Net income	$ 93,214	$ 70,514	$ 291,370	$ 224,979
Adjustments to reconcile net income to net cash provided by operating activities:
Benefit of (provision for) deferred income taxes			12,850	(438)
All other adjustments to reconcile net income to net cash provided by operating activities			86,536
Changes in assets and liabilities, net			(126,320)
Net cash provided by operating activities			264,436	$ 252,150
Previous U.S. GAAP
Cash Flows from Operating Activities:
Net income	88,822		277,699
Adjustments to reconcile net income to net cash provided by operating activities:
Benefit of (provision for) deferred income taxes			9,244
All other adjustments to reconcile net income to net cash provided by operating activities			86,536
Changes in assets and liabilities, net			(109,043)
Net cash provided by operating activities			264,436
Attributed to the New Revenue Standard | Accounting Standards Update 2014-09
Cash Flows from Operating Activities:
Net income	$ 4,392		13,671
Adjustments to reconcile net income to net cash provided by operating activities:
Benefit of (provision for) deferred income taxes			3,606
All other adjustments to reconcile net income to net cash provided by operating activities			0
Changes in assets and liabilities, net			(17,277)
Net cash provided by operating activities			$ 0