Dividends - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		6 Months Ended
	Jul. 18, 2018	Apr. 24, 2018	Jun. 17, 2018	Jun. 18, 2017	Jun. 17, 2018	Jun. 18, 2017	Dec. 31, 2017	[1]
Dividends Payable [Line Items]
Dividends declared per share		$ 0.55	$ 0.55	$ 0.46	$ 1.10	$ 0.92
Common stock dividends accrued			$ 23,559		$ 23,559		$ 536
Dividend declared date		Apr. 24, 2018
Record date of dividend		Jun. 15, 2018
Dividend payable date		Jun. 29, 2018
Subsequent Event [Member]
Dividends Payable [Line Items]
Dividends declared per share	$ 0.55
Dividend declared date	Jul. 18, 2018
Record date of dividend	Sep. 14, 2018
Dividend payable date	Sep. 28, 2018

[1] Note: The balance sheet at December 31, 2017 has been derived from the audited consolidated financial statements at that date but does not include all of the information and footnotes required by accounting principles generally accepted in the United States for complete financial statements.