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Capital Structure - Narrative (Details) - USD ($) |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Sep. 20, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016 | Feb. 08, 2017
Class of Stock [Line Items] |  |  |  |  |  |  |
Additional paid-in capital |  | $ 951,800,000 |  | $ 951,800,000 |  | $ 1,259,600,000 |
Aggregate liquidation preferences |  | $ 525,000,000 |  | 525,000,000 |  | 818,200,000 |
Issuance costs |  |  |  | $ 13,100,000 |  | $ 21,100,000 |
Shares repurchase and cancelled during period (in shares) |  | 451,268 | 144,289 | 648,941 | 1,122,328 |  |
Shares repurchase and cancelled during period, value |  | $ 20,000,000 | $ 6,500,000 | $ 30,000,000 | $ 50,000,000 |  |
Average price (in usd per share) |  | $ 44.32 | $ 45.17 | $ 46.23 | $ 44.55 |  |
Preferred shares, liquidation preference (in USD per share) |  | $ 25 |  | $ 25 |  | $ 25 |
Repurchase Program 2017 |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Existing share repurchase authorization amount |  |  |  |  |  |  | $ 250,000,000
5.625% preference shares |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Issuance costs | $ 8,700,000 |  |  |  |  |  |
Preference shares, issued (in shares) | 10,000,000 | 10,000,000 |  | 10,000,000 |  | 10,000,000 |
Preference shares, rate |  |  |  | 5.625% |  | 5.625% |
Preferred shares, liquidation preference (in USD per share) | $ 25 | $ 25 |  | $ 25 |  | $ 25 |
Net proceeds from issuance of preference shares | $ 241,300,000 |  |  |  |  |  |
Preferred shares, liquidation preference, value | $ 250,000,000 |  |  |  |  |  |
7.401% preference shares |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Preference shares, issued (in shares) |  | 0 |  | 0 |  | 5,327,500 |
Preference shares, rate |  |  |  | 7.401% |  | 7.401% |
Preferred shares, liquidation preference (in USD per share) |  |  |  |  |  | $ 25 |
7.250% preference shares |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Preference shares, issued (in shares) |  | 0 |  | 0 |  | 6,400,000 |
Preference shares, rate |  |  |  | 7.25% |  | 7.25% |
Preferred shares, liquidation preference (in USD per share) |  |  |  |  |  | $ 25 |

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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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