Capital Stock - Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Class of Stock [Line Items]
Dividends on preferred stock	$ 9,400	$ 9,400	$ 18,800	$ 14,683
Preferred Class A
Class of Stock [Line Items]
Dividends on preferred stock	$ 3,000	$ 3,000	$ 6,000	$ 6,000
Preferred stock dividends declared (in USD per share)	$ 0.5	$ 0.50	$ 1	$ 1.00
Preferred Class B
Class of Stock [Line Items]
Dividends on preferred stock	$ 7,000	$ 7,000	$ 13,000	$ 9,000
Preferred stock dividends declared (in USD per share)	$ 0.5	$ 0.5	$ 0.9999910769	$ 0.68333