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Employee Benefit Plans, 401(k) Plan and Other Retirement Benefits (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Other Retirement Benefits [Abstract] |  |  |
Supplemental retirement benefits for retired executives under other retirement benefits plan | $ 2.4 | $ 2.6 |
Expense related to other retirement benefits plan | 0.1 | 0.2 | $ 0.3
401(K) Plan [Member] |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Employer contribution to plan | $ 2.8 | $ 2.7 | $ 2.5

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- Definition Amount of cost (reversal of cost) for other postretirement benefits. Excludes pension benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(b)(4)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(b)(2)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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