Capital Stock (Details)				3 Months Ended						9 Months Ended		12 Months Ended
	Nov. 08, 2018 $ / shares	Feb. 03, 2017 USD ($) $ / shares	Jan. 02, 2017 $ / shares	Sep. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Feb. 24, 2016 USD ($)	Dec. 31, 2012 shares
Class of Stock [Line Items]
Preferred stock authorized (shares)				40,000,000						40,000,000		40,000,000
Preferred stock par value (usd per share) | $ / shares				$ 0.001						$ 0.001		$ 0.001
Treasury stock (shares)				14,102,185						14,102,185		14,102,185
Dividends declared to shareholders and warrant holders | $				$ 21,929,000	$ 21,925,000	$ 21,922,000	$ 21,834,000	$ 21,816,000	$ 21,742,000	$ 65,776,000	$ 65,392,000
Special Cash Dividend
Class of Stock [Line Items]
Dividends declared per common share (usd per share) | $ / shares			$ 5.77
Warrant, Dividends, Per Share, Cash Paid | $ / shares		$ 5.77
Common stock dividends (usd per share) | $ / shares		$ 5.77
Dividends declared to shareholders and warrant holders | $		$ 499,000,000
Regular Cash Dividend
Class of Stock [Line Items]
Dividends declared per common share (usd per share) | $ / shares				$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.75	$ 0.75
Regular Cash Dividend | Subsequent Event
Class of Stock [Line Items]
Dividends declared per common share (usd per share) | $ / shares	$ 0.25
Warrant
Class of Stock [Line Items]
Warrants issued (shares)				30,551						30,551				16,789,972
Number of shares called by each warrant (shares)				1						1
Warrant, exercise price, per share (usd per share) | $ / shares				$ 0.001						$ 0.001
Class A Common Stock
Class of Stock [Line Items]
Common stock, authorized (shares)				1,000,000,000						1,000,000,000		1,000,000,000
Common stock, par value (usd per share) | $ / shares				$ 0.001						$ 0.001		$ 0.001
Common stock, conversion ratio										1
Number of warrants exercised (shares)							46,802			0	91,650
Number of shares received							46,802				91,650
Number of shares of common stock issued (shares)				101,745,449						101,745,449		101,429,999
Shares repurchases authorized (amount) | $													$ 400,000,000
Amount of stock repurchased | $										$ 0		$ 0
Remaining authorized repurchase amount | $				$ 168,000,000						$ 168,000,000
Class B Common Stock
Class of Stock [Line Items]
Common stock, authorized (shares)				1,000,000,000						1,000,000,000		1,000,000,000
Common stock, par value (usd per share) | $ / shares				$ 0.001						$ 0.001		$ 0.001
Common stock, conversion ratio										1
Number of shares converted (shares)										0	0
Number of warrants exercised (shares)										0	0
Number of shares of common stock issued (shares)				5,557						5,557		5,557
Preferred Stock
Class of Stock [Line Items]
Preferred stock authorized (shares)				40,000,000						40,000,000
Preferred stock par value (usd per share) | $ / shares				$ 0.001						$ 0.001