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
	Mar. 31, 2018	Mar. 31, 2017
Cash Flows from Operating Activities:
Net income	$ 89,476	$ 69,058
Adjustments to reconcile net income to net cash provided by operating activities:
Benefit of deferred income taxes	3,005	1,941
Net cash provided by operating activities	34,898	$ 31,274
Accounting Standards Update 2014-09 [Member]
Cash Flows from Operating Activities:
Net income	89,476
Adjustments to reconcile net income to net cash provided by operating activities:
Benefit of deferred income taxes	3,005
All other adjustments to reconcile net income to net cash provided by operating activities	27,855
Changes in assets and liabilities, net	(85,438)
Net cash provided by operating activities	34,898
Attributed to the New Revenue Standard [Member] | Accounting Standards Update 2014-09 [Member]
Cash Flows from Operating Activities:
Net income	4,539
Adjustments to reconcile net income to net cash provided by operating activities:
Benefit of deferred income taxes	1,917
Changes in assets and liabilities, net	(6,456)
Previous U.S. GAAP (Reported) [Member] | Accounting Standards Update 2014-09 [Member]
Cash Flows from Operating Activities:
Net income	84,937
Adjustments to reconcile net income to net cash provided by operating activities:
Benefit of deferred income taxes	1,088
All other adjustments to reconcile net income to net cash provided by operating activities	27,855
Changes in assets and liabilities, net	(78,982)
Net cash provided by operating activities	$ 34,898