Shareholders' Equity - Narrative (Details)							1 Months Ended		12 Months Ended
	Oct. 23, 2018 USD ($)	Oct. 09, 2018 $ / shares	Mar. 14, 2018 USD ($)	Mar. 15, 2017 USD ($)	May 15, 2016 USD ($) $ / shares shares	Mar. 17, 2016 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) $ / shares shares	Oct. 10, 2018
Class of Stock [Line Items]
Redemption of shares											$ 275,000,000
Distributions declared per common share (in dollars per share) | $ / shares		$ 2.5
Cash distributions paid	$ 304,700,000									$ 0	0
Preferred stock redemption premium									$ 0	$ 0	$ 9,609,000
Distribution percentage to shareholders consisting of an alternative minimum tax adjustment										0.00%	96.31%
Common Shares
Class of Stock [Line Items]
Stock repurchase program, additional authorized amount			$ 150,000,000	$ 150,000,000.0		$ 150,000,000.0
Stock repurchase program, period in force			12 months	12 months		12 months
Stock repurchase program, amount not utilized							$ 81,000,000	$ 106,600,000
Number of shares repurchased (in shares) | shares									2,970,209	0	2,491,675
Stock repurchased and retired during period, price per share (in dollars per share) | $ / shares									$ 29.67		$ 27.68
Redemption of shares			$ 19,100,000	$ 69,000,000					$ 88,100,000		$ 69,000,000
Stock repurchase program, remaining authorized repurchase amount									$ 130,900,000
Stock repurchase program, number of shares surrendered (in shares) | shares									193,521	43,329	31,025
Common shares from conversion of preferred shares (in shares) | shares									2,563,275
Series D
Class of Stock [Line Items]
Number of shares repurchased (in shares) | shares											0
Redemption of shares											$ 0
Preferred shares, of beneficial interest, shares outstanding (in shares) | shares									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										0.5215
Initial conversion price per share (in dollars per share) | $ / shares									$ 47.9386385427
Additional common shares (in shares) | shares									2,563,275
Liquidation preference (in dollars per share) | $ / shares									$ 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
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)