Common Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Nov. 30, 2018	May 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2017	Nov. 30, 2014
Equity, Class of Treasury Stock [Line Items]
Treasury stock acquired, value			$ 23
Cash dividends declared per share							$ 0.50
Dividends payable							$ 21,900
Dividend paid in cash				$ 21,900		$ 21,911
Callcap Acquisition
Equity, Class of Treasury Stock [Line Items]
Business acquisition, percentage of voting interests acquired	100.00%
Cash paid for business acquisition	$ 24,993
Class B
Equity, Class of Treasury Stock [Line Items]
Number of shares authorized to be repurchased								3,000,000
Treasury stock acquired, shares					90,000	0
Treasury stock repurchased					$ 0.01
Treasury stock acquired, value					$ 900	$ 0
Class B | Callcap Acquisition
Equity, Class of Treasury Stock [Line Items]
Stock issued during the period, shares	3,400,000
Common stock issuance period	4 years
Class B | Former Member of Board of Directors
Equity, Class of Treasury Stock [Line Items]
Treasury stock acquired, shares		2,300,000
Treasury stock acquired, value		$ 5,700