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Capital Structure - Additional Information (Details) - USD ($) |  |  | 1 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jan. 03, 2017 | Sep. 20, 2016 | Jul. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jul. 03, 2017 | Feb. 08, 2017 | Feb. 05, 2015
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Additional paid in capital |  |  |  | $ 954,700,000 | $ 1,259,600,000 |  |  |  |
Aggregate liquidation preferences |  |  |  | 525,000,000 | 818,200,000 |  |  |  |
Preferred stock issuance cost |  |  |  | 13,100,000 | 21,100,000 |  |  |  |
Stock repurchase program, remaining authorized repurchase amount |  |  |  | $ 220,000,000 | $ 341,300,000 | $ 416,300,000 |  |  |
Preferred Stock, Redemption Amount | $ 133,200,000 |  |  |  |  |  | $ 160,000,000 |  |
Redemption price per share |  |  |  | $ 25 | $ 25 |  |  |  |
Preference shares liquidation preference, value |  |  |  | $ 250,000,000 |  |  |  |  |
5.625% Preference Shares |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Preferred stock issuance cost |  |  |  | $ 8,700,000 |  |  |  |  |
Preference shares, issued |  |  |  | 10,000,000 | 10,000,000 |  |  |  |
Preference shares, rate |  | 5.625% |  | 5.625% | 5.625% |  |  |  |
Redemption price per share |  |  |  | $ 25 | $ 25 |  |  |  |
7.401% Preference Shares |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Preference shares, issued |  |  |  | 0 | 5,327,500 |  |  |  |
Preference shares, rate | 7.401% |  |  | 7.401% | 7.401% |  |  |  |
Redemption price per share |  |  |  |  | $ 25 |  | $ 25 |  |
7.250% Preference Shares |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Preference shares, issued |  |  |  | 0 | 6,400,000 |  |  |  |
Preference shares, rate |  |  | 7.25% | 7.25% | 7.25% |  |  |  |
Redemption price per share |  |  |  |  | $ 25 |  |  |  |
Open Markets |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Stock repurchased during period, shares |  |  |  | 648,941 | 1,595,076 | 1,790,333 |  |  |
Stock repurchased during period, value |  |  |  | $ 30,000,000 | $ 75,000,000 | $ 83,700,000 |  |  |
Stock repurchased during period, average price per share |  |  |  | $ 46.23 | $ 47.02 | $ 46.74 |  |  |
Repurchase Program 2015 |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Stock repurchase program, authorized amount |  |  |  |  |  |  |  |  | $ 500,000,000
Repurchase Program 2017 |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Stock repurchase program, authorized amount |  |  |  |  |  |  |  | $ 250,000,000.0 |
Additional paid-in capital | Preference Shares |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Shares issued |  |  |  | $ 0 | $ 241,300,000 | $ 0 |  |  |

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- Definition Excess of issue price over par or stated value of the entity's capital stock and amounts received from other transactions involving the entity's stock or stockholders. Includes adjustments to additional paid in capital. Some examples of such adjustments include recording the issuance of debt with a beneficial conversion feature and certain tax consequences of equity instruments awarded to employees. Use this element for the aggregate amount of additional paid-in capital associated with common and preferred stock. For additional paid-in capital associated with only common stock, use the element additional paid in capital, common stock. For additional paid-in capital associated with only preferred stock, use the element additional paid in capital, preferred stock. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(30)(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The per share liquidation preference (or restrictions) of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21484-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(d)(1)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(d)(1)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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