Shareholders' Equity - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2014	May 08, 2018
Stockholders Equity Note Disclosure [Line Items]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.18	$ 0.35
ApprovedNotDeclaredlDividends			$ 0.72
Dividends payable	$ 30,900,000		$ 30,800,000
December 2014 Share Repurchase Program [Member]
Stockholders Equity Note Disclosure [Line Items]
Share repurchase program remaining available amount	$ 400,000,120
May 2018 Share Repurchase Program [Member]
Stockholders Equity Note Disclosure [Line Items]
Repurchase of shares of our common stock up to a maximum aggregate value					$ 750,000,000
Common stock authorized for repurchase, expiration date				May 31, 2021