Stockholders' Equity (Narrative) (Details) $ / shares in Units, $ in Thousands				9 Months Ended
	Mar. 02, 2020 USD ($)	Feb. 28, 2020 director quarter $ / shares shares	Apr. 15, 2013 $ / shares shares	Sep. 30, 2021 USD ($) quarter director $ / shares shares	Sep. 30, 2020 USD ($) $ / shares	Dec. 31, 2020 $ / shares shares
Stockholders' Equity
Proceeds from issuance of redeemable preferred stock | $				$ 0	$ 275,000
Series B Preferred Stock
Stockholders' Equity
Preferred Stock, dividend rate				7.50%
Preferred stock, shares issued (in shares) | shares				8,000,000		8,000,000
Preferred stock par value (usd per share)				$ 0.01		$ 0.01
Preferred stock, shares outstanding (in shares) | shares				8,000,000		8,000,000
Series C Preferred Stock
Stockholders' Equity
Preferred Stock, dividend rate				6.50%
Preferred stock, shares issued (in shares) | shares				11,000,000		11,000,000
Preferred stock par value (usd per share)				$ 0.01		$ 0.01
Preferred stock, shares outstanding (in shares) | shares				11,000,000		11,000,000
Preferred Stock | Series B Preferred Stock
Stockholders' Equity
Preferred Stock, dividend rate			7.50%	7.50%	7.50%
Preferred stock, shares issued (in shares) | shares			8,000,000
Preferred stock par value (usd per share)			$ 0.01
Preferred stock, liquidation preference (in dollars per share)			25.00	$ 25.00	$ 25.00
Preferred stock, redemption price (in dollars per share)			$ 25.00
Maximum quarters without dividends to get voting rights, in quarters | quarter				6
Number of additional directors that can be elected by preferred stock holders | director				2
Percentage required for making material changes			66.67%
Preferred Stock | Series C Preferred Stock
Stockholders' Equity
Preferred Stock, dividend rate		6.50%		6.50%	6.50%
Preferred stock, shares issued (in shares) | shares		11,000,000
Preferred stock par value (usd per share)		$ 0.01
Preferred stock, liquidation preference (in dollars per share)		25.00		$ 25.00	$ 25.00
Preferred stock, redemption price (in dollars per share)		$ 25.00
Maximum quarters without dividends to get voting rights, in quarters | quarter		6
Number of additional directors that can be elected by preferred stock holders | director		2
Shares reclassified to redeemable capital stock (in shares) | shares		12,650,000
Proceeds from issuance of redeemable preferred stock | $	$ 266,000
Percentage required for making material changes			66.67%
LIBOR | Preferred Stock | Series C Preferred Stock
Stockholders' Equity
Debt instrument, basis spread on variable rate		5.345%