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Shareholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Feb. 28, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Jun. 30, 2017 | Mar. 31, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2015 | Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Authorized common stock (in shares) |  | 100,000,000 |  |  |  | 100,000,000 |  |  | 100,000,000
Authorized preferred stock (in shares) |  | 50,000,000 |  |  |  | 50,000,000 |  |  | 50,000,000
Quarterly dividend declared, per share (USD per share) |  | $ 0.31 | $ 0.310 | $ 0.31 | $ 0.310 | $ 0.62 | $ 0.62 |  |
Dividend declared |  |  |  |  |  | $ 34.2 | $ 33.0 |  |
Total authorizations which remain available for use |  | $ 109.6 |  |  |  | $ 109.6 |  |  |
Common shares acquired (in shares) |  |  |  |  |  | 0 | 0 |  |
Bonus compensation shares issued (in shares) | 2,500 |  |  |  |  |  |  |  |
Non-credit loss, net of tax |  | $ 0.6 |  |  |  | $ 0.6 |  |  | $ 0.5
Restricted Share Units |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Awards in period (in shares) | 85,800 |  |  |  |  |  |  |  |
Fair value of each unit awarded (USD per share) | $ 44.73 |  |  |  |  |  |  |  |
Award vesting period |  |  |  |  |  | 3 years |  |  |
Shares issued in period for share-based compensation (in shares) | 52,800 |  |  |  |  |  |  |  |
Performance Share Units |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Awards in period (in shares) | 27,200 |  |  |  |  |  |  |  |
Shares issued in period for share-based compensation (in shares) | 80,600 |  |  |  |  |  |  |  |
Performance Share Units | Minimum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award payout rate |  |  |  |  |  | 50.00% |  |  |
Performance Share Units | Maximum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award payout rate |  |  |  |  |  | 200.00% |  |  |
Performance Shares Granted in 2015 | Maximum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award payout rate |  |  |  |  |  |  |  | 125.00% |
Accumulated Other Comprehensive Income (Loss) |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Reclassification from AOCI to retained earnings for stranded tax effects |  |  |  |  |  | $ 3.4 |  |  |
Retained Earnings |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Reclassification from AOCI to retained earnings for stranded tax effects |  |  |  |  |  | $ (3.4) |  |  |

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