Shareholders' Equity - Narrative (Details)					1 Months Ended	12 Months Ended
	Jun. 15, 2017 USD ($) director $ / shares shares	Sep. 15, 2016 shares	Nov. 25, 2015 USD ($) director $ / shares shares	Aug. 22, 2012 USD ($) director $ / shares shares	Oct. 31, 2013 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Feb. 21, 2017 USD ($)	Dec. 31, 2015 shares
Class of Stock [Line Items]
Shares authorized (in shares) | shares						150,000,000
Common shares, shares issued (in shares) | shares						87,938,537	87,730,054
Common shares, shares outstanding (in shares) | shares						82,948,577	82,974,895	86,271,109		73,721,140
Preference shares issued (in shares) | shares						18,600,000
Shares undesignated (in shares) | shares						43,461,463
Common shares, par value (in dollars per share)						$ 0.01	$ 0.01
Dividends on common stock (in dollars per share)						$ 0.35	$ 0.60	$ 0.57
Number of shares issued (in shares) | shares						198,983	246,382	251,027
Proceeds from issuance of preference shares | $						$ 0	$ 144,942,000	$ (143,000)
Preferred Stock, number of directors contingently electable | director				2
Preferred stock, dividend default period				six or more dividend periods
Authorized amount of share repurchase program | $									$ 100,000,000
Shares repurchased (in shares) | shares						205,000	3,667,134
Shares repurchased, price per share (in dollars per share)						$ 3.31	$ 6.84
Remaining shares authorized | $						$ 74,245,000	$ 74,924,000
Shares repurchased for tax withholding (in shares) | shares						29,801	38,122	35,258
Shares repurchased for tax withholding for tax withholding (in dollars per share)						$ 6.52	$ 15.06	$ 13.33
Preference shares - Series D
Class of Stock [Line Items]
Number of shares issued (in shares) | shares	6,000,000
Dividend rate of preference shares	6.70%
Par value of preference shares (in dollars per share)	$ 0.01
Fair value of share (in dollars per share)	$ 25
Proceeds from issuance of preference shares | $	$ 144,942,000
Expenses and underwriting discounts | $	$ 5,058,000
Preferred Stock, number of directors contingently electable | director	2
Dividends on preference shares | $	$ 5,025,000					$ 7,538,000
Dividends on preference shares (in dollars per share)						$ 1.2563	$ 0.8375
Redemption price for preference shares (in dollars per share)	$ 25
Preference Shares - Series C
Class of Stock [Line Items]
Number of shares issued (in shares) | shares			6,600,000
Dividend rate of preference shares			7.125%
Par value of preference shares (in dollars per share)			$ 0.01
Fair value of share (in dollars per share)			$ 25
Proceeds from issuance of preference shares | $			$ 159,485,000
Expenses and underwriting discounts | $			$ 5,515,000
Sale price of preference shares (in dollars per share)			$ 25
Preferred Stock, number of directors contingently electable | director			2
Dividends on preference shares | $						$ 8,816,000	$ 11,756,000	$ 12,410,000
Dividends on preference shares (in dollars per share)						$ 1.3359	$ 1.7813	$ 1.8803
Redemption price for preference shares (in dollars per share)			$ 25
Mandatory Convertible Preference Shares - Series B
Class of Stock [Line Items]
Dividend rate of preference shares					7.25%
Par value of preference shares (in dollars per share)					$ 0.01
Fair value of share (in dollars per share)					$ 50
Proceeds from issuance of preference shares | $					$ 159,675,000
Expenses and underwriting discounts | $					$ 5,325,000
Sale price of preference shares (in dollars per share)					$ 50
Dividends on preference shares | $								$ 8,971,000
Dividends on preference shares (in dollars per share)								$ 2.7188
Preference shares offered (in shares) | shares					3,300,000
Liquidation preference per share per annum (in dollars per share)					$ 3.625
Liquidation preference per share per quarter (in dollars per share)					0.90625
Liquidation preference per share in initial quarter (in dollars per share)					$ 0.745139
Convertible preferred stock, conversion rate measurement period					40 days
Series B Convertible Preference Shares
Class of Stock [Line Items]
Number of shares converted (in shares) | shares		12,069,090				0	0	12,069,090
Common shares issued per convertible preference share (in shares) | shares		3.6573
Preference shares – Series A
Class of Stock [Line Items]
Number of shares issued (in shares) | shares				6,000,000
Dividend rate of preference shares				8.25%
Par value of preference shares (in dollars per share)				$ 0.01
Fair value of share (in dollars per share)				$ 25
Proceeds from issuance of preference shares | $				$ 145,041,000
Expenses and underwriting discounts | $				$ 4,959,000
Sale price of preference shares (in dollars per share)				$ 25
Dividends on preference shares | $						$ 9,282,000	$ 12,375,000	$ 12,375,000
Dividends on preference shares (in dollars per share)						$ 1.5469	$ 2.0625	$ 2.0625
Redemption price for preference shares (in dollars per share)				$ 25