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Financial Instruments and Fair Value Measurement - Equity Securities (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2015 | Sep. 30, 2015 | Mar. 31, 2016
Available-for-sale securities |  |  |
Available-for-sale securities in unrealized loss positions, number of days | 30 days | 30 days |
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Non-qualified Supplemental Savings Plan |  |  |
Assets held in Non-qualified Supplement Savings Plan, at fair value |  | $ 12,900 | $ 12,200
Equity securities |  |  |
Available-for-sale securities |  |  |
Cost |  | 64,462 | 64,462
Gross Unrealized Gains |  | 28,530 | 30,765
Gross Unrealized Losses |  | 1,509 | 24,069
Estimated Fair Value |  | $ 91,483 | $ 71,158

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