Shareholders' Equity - Narrative (Details)				1 Months Ended	2 Months Ended	12 Months Ended			28 Months Ended
	May 15, 2016 USD ($) $ / shares shares	Mar. 17, 2016 USD ($)	Aug. 24, 2015 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($) authorization	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Mar. 15, 2017 USD ($)	Sep. 14, 2015 USD ($)
Class of Stock [Line Items]
Stock repurchase program, period in force		12 months	12 months
Stock repurchase program, number of authorizations expired | authorization					2
Stock repurchase program, amount not utilized				$ 106,600,000	$ 86,600,000
Redemption of shares							$ 275,000,000	$ 0
Preferred stock redemption premium						$ 0	$ 9,609,000	$ 0
Distribution percentage to shareholders consisting of an alternative minimum tax adjustment						0.00%	96.31%	0.00%
Common Shares
Class of Stock [Line Items]
Common shares issued to holders of preferred shares (in shares) | shares								144
Number of shares repurchased (in shares) | shares							2,491,675	3,410,300	5,901,975
Stock repurchase program, authorized amount			$ 100,000,000.0
Stock repurchase program, additional authorized amount		$ 150,000,000.0								$ 150,000,000.0	$ 100,000,000.0
Stock repurchased and retired during period, price per share (in dollars per share) | $ / shares							$ 27.68	$ 25.76	$ 26.57
Redemption of shares							$ 69,000,000	$ 87,800,000	$ 156,800,000
Stock repurchase program, remaining authorized repurchase amount						$ 150,000,000			$ 150,000,000
Stock repurchase program, number of shares surrendered (in shares) | shares						43,329	31,025
Common shares from conversion of preferred shares (in shares) | shares						2,363,103			2,363,103
Series D
Class of Stock [Line Items]
Number of shares repurchased (in shares) | shares							0	301
Redemption of shares							$ 0	$ 3,000
Preferred shares, of beneficial interest, shares outstanding (in shares) | shares						4,915,196	4,915,196		4,915,196
Preferred shares dividend (in dollars per share) | $ / shares						$ 1.625
Preferred shares, dividend yield						6.50%	6.50%
Initial conversion rate						0.480775			0.480775
Initial conversion price per share (in dollars per share) | $ / shares						$ 52.00			$ 52.00
Additional common shares (in shares) | shares						2,363,103
Liquidation preference (in dollars per share) | $ / shares						$ 25.00			$ 25.00
Percentage of average closing market price of the entity's common stock based on which ratio for conversion of preferred shares into common shares is determined						98.00%
Purchase price as a percentage of liquidation preference						100.00%
Common shares from conversion of preferred shares (in shares) | shares						4,915,196			4,915,196
Series E
Class of Stock [Line Items]
Number of shares repurchased (in shares) | shares							11,000,000
Redemption of shares							$ 275,000,000
Stock redeemed during period (in shares) | shares	11,000,000
Redemption price (in dollars per share) | $ / shares	$ 25.00
Preferred stock, redemption amount	$ 275,000,000.0
Preferred stock redemption premium							$ (9,609,000)