Stockholder's Equity (Narrative) (Details) $ / shares in Units, $ in Thousands		3 Months Ended		12 Months Ended
	Apr. 30, 2018 trading_day tracking_days	Mar. 31, 2012 shares	Mar. 31, 2011 $ / shares shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)
Preferred Stock [Line Items]
Preferred stock issued (in shares)				14,375,000	14,375,000
Common stock conversion, threshold percentage of daily volume-weighted average prices of common stock minimum	130.00%
Common stock conversion, threshold trading days | trading_day	20
Common stock conversion, threshold consecutive tracking days | tracking_days	30
Preferred stock redemption charge | $				$ 0	$ 9,769	$ 0
Series I Preferred Stock
Preferred Stock [Line Items]
Preferred stock issued (in shares)			14,375,000
Preferred stock balance issued average rate			6.50%
Liquidation value per preferred share (in USD per share) | $ / shares			$ 50.00
Equivalent number of common stock on conversion of preferred shares (in shares)			0.8460
Initial conversion Price (in USD per share) | $ / shares			$ 59.10
Series J Preferred Stock
Preferred Stock [Line Items]
Preferred stock issued (in shares)		11,500,000
Preferred stock balance issued average rate		6.50%
Preferred stock redemption charge | $					$ 9,769