Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Stockholders Equity [Line Items]
Common stock held by subsidiaries, shares		10,747,142				7,628,786				10,747,142	7,628,786
Shares repurchased, value										$ 306,741	$ 300,217	$ 172,772
Share repurchase remaining authorization		$ 248,384								$ 248,384
Preferred stock, shares authorized		15,000,000				15,000,000				15,000,000	15,000,000
Preferred stock, par value		$ 0.01				$ 0.01				$ 0.01	$ 0.01
Conversion of Series A preferred stock into Class A common stock												0
Dividend declared per share of common stock		$ 0.41	$ 0.41	$ 0.41	$ 1.58	$ 0.38	$ 0.38	$ 0.38	$ 1.55	$ 2.81	$ 2.69	$ 2.35
Subsequent Event [Member]
Schedule Of Stockholders Equity [Line Items]
Dividend declared per share of common stock	$ 1.71
Dividend payable date	Feb. 23, 2018
Dividend date of record	Feb. 12, 2018
Dividend declare date	Jan. 31, 2018
Subsequent Event [Member] | Quarterly Dividend [Member]
Schedule Of Stockholders Equity [Line Items]
Dividend declared per share of common stock	$ 0.41
Subsequent Event [Member] | Extra Cash Dividend [Member]
Schedule Of Stockholders Equity [Line Items]
Dividend declared per share of common stock	$ 1.30
November, 2016 [Member]
Schedule Of Stockholders Equity [Line Items]
Share repurchase remaining authorization		$ 48,384								$ 48,384
Share repurchase authorization expiration date										Dec. 31, 2018
October, 2017 [Member]
Schedule Of Stockholders Equity [Line Items]
Share repurchase remaining authorization		$ 200,000								$ 200,000
Share repurchase authorization expiration date										Dec. 31, 2019
Class A Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Common stock held by subsidiaries, shares		10,747,142				7,628,786				10,747,142	7,628,786
Class A Common Stock [Member] | Executive Officers [Member]
Schedule Of Stockholders Equity [Line Items]
Shares repurchased, value										$ 14,700	$ 4,900	$ 17,700
Series A Preferred Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Preferred stock, shares outstanding		0				0				0	0	7,921
Series B Preferred Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Preferred stock, shares outstanding		0				0				0	0	0