General Information - Retrospective Effect of Adoption that Impacted on Balance Sheets (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Jan. 01, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 1,996,438	$ 1,946,131	$ 1,946,765
Additional paid-in capital	$ 2,009,953	2,005,148	$ 2,004,514
Scenario, Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		1,946,765
Additional paid-in capital		2,004,514
Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		(634)
Additional paid-in capital		$ 634