Unaudited Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions |
Sep. 30, 2016 |
Dec. 31, 2015 |
|---|---|---|
| Accumulated depreciation on vessels and equipment | $ 449.6 | $ 391.0 |
| Common stock, shares authorized (shares) | 300,000,000 | 300,000,000 |
| Class A [Member] | ||
| Common stock, shares authorized (shares) | 200,000,000 | 200,000,000 |
| Common stock, shares issued (shares) | 133,100,000 | 132,800,000 |
| Common stock, shares outstanding (shares) | 133,100,000 | 132,800,000 |
| Class B [Member] | ||
| Common stock, shares authorized (shares) | 100,000,000 | 100,000,000 |
| Common stock, shares issued (shares) | 23,200,000 | 23,200,000 |
| Common stock, shares outstanding (shares) | 23,200,000 | 23,200,000 |