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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Cash Flows from Operating Activities:
Net income	$ 108,680	$ 85,407	$ 198,156	$ 154,465
Adjustments to reconcile net income to net cash provided by operating activities:
Benefit of deferred income taxes			8,638	2,691
Net cash provided by operating activities			153,728	$ 141,466
Accounting Standards Update 2014-09
Cash Flows from Operating Activities:
Net income	108,680		198,156
Adjustments to reconcile net income to net cash provided by operating activities:
Benefit of deferred income taxes			8,638
All other adjustments to reconcile net income to net cash provided by operating activities			55,661
Changes in assets and liabilities, net			(108,727)
Net cash provided by operating activities			153,728
Previous U.S. GAAP | Accounting Standards Update 2014-09
Cash Flows from Operating Activities:
Net income	103,940		188,877
Adjustments to reconcile net income to net cash provided by operating activities:
Benefit of deferred income taxes			6,218
All other adjustments to reconcile net income to net cash provided by operating activities			55,661
Changes in assets and liabilities, net			(97,028)
Net cash provided by operating activities			153,728
Attributed to the New Revenue Standard | Accounting Standards Update 2014-09
Cash Flows from Operating Activities:
Net income	$ 4,740		9,279
Adjustments to reconcile net income to net cash provided by operating activities:
Benefit of deferred income taxes			2,420
All other adjustments to reconcile net income to net cash provided by operating activities			0
Changes in assets and liabilities, net			(11,699)
Net cash provided by operating activities			$ 0