Capital Stock	12 Months Ended
Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Capital Stock	CAPITAL STOCK CAPITAL STOCK At December 31, 2017, our authorized capital stock consists of 200,000,000 shares of common stock, par value $0.01 per share, and 10,000,000 shares of preferred stock, par value $0.01 per share. TREASURY STOCK On October 17, 2016, our Board of Directors authorized the repurchase by us of up to an additional $300.0 million of our common stock by various means from time to time through and including December 31, 2021. Pursuant to this authorization, together with the remaining amount from a prior authorization, we repurchased a total of 294,930 shares of our common stock in 2016 at an average price per share of $24.40, for a total purchase price of $7.2 million, and we completed a “modified Dutch auction” tender offer in 2016 pursuant to which we purchased 10,617,810 shares of our common stock from our stockholders at a price of $25.38 per share, for a total purchase price of $269.4 million, exclusive of $0.7 million of fees and expenses. Certain of our directors and executive officers as well as related family foundations participated in the 2016 "modified Dutch auction" tender offer, and we purchased a total of 3,358,572 shares of our common stock beneficially owned by them at a price of $25.38 per share, for a total purchase price of $85.2 million, in such tender offer. In 2015, we repurchased a total of 278,842 shares of our common stock at an average price per share of $27.36, for a total purchase price of $7.6 million, and we completed a “modified Dutch auction” tender offer pursuant to which we purchased 5,532,708 shares of our common stock from our stockholders at a price of $29.25 per share, for a total purchase price of $161.8 million, exclusive of $0.7 million of fees and expenses. At December 31, 2017, we had approximately $129.4 million remaining for the repurchase of our common stock under the October 17, 2016 Board of Directors authorization. In 2017, 2016 and 2015, we issued 421,108 treasury shares, 363,368 treasury shares and 292,320 treasury shares, respectively, at an average cost of $3.15 per share, for restricted stock units that vested during these years. In 2017, 2016 and 2015, we repurchased 138,080 shares, 134,122 shares and 101,682 shares of our common stock, respectively, at an average cost of $30.24, $25.58 and $28.44 per share, respectively, in accordance with the Plan to satisfy employee withholding tax requirements resulting from certain restricted stock units becoming vested. We account for treasury shares using the FIFO cost method. As of December 31, 2017, 64,727,152 shares of our common stock were held in treasury.

CAPITAL STOCK

At December 31, 2017, our authorized capital stock consists of 200,000,000 shares of common stock, par value $0.01 per share, and 10,000,000 shares of preferred stock, par value $0.01 per share.

TREASURY STOCK

On October 17, 2016, our Board of Directors authorized the repurchase by us of up to an additional $300.0 million of our common stock by various means from time to time through and including December 31, 2021. Pursuant to this authorization, together with the remaining amount from a prior authorization, we repurchased a total of 294,930 shares of our common stock in 2016 at an average price per share of $24.40, for a total purchase price of $7.2 million, and we completed a “modified Dutch auction” tender offer in 2016 pursuant to which we purchased 10,617,810 shares of our common stock from our stockholders at a price of $25.38 per share, for a total purchase price of $269.4 million, exclusive of $0.7 million of fees and expenses. Certain of our directors and executive officers as well as related family foundations participated in the 2016 "modified Dutch auction" tender offer, and we purchased a total of 3,358,572 shares of our common stock beneficially owned by them at a price of $25.38 per share, for a total purchase price of $85.2 million, in such tender offer. In 2015, we repurchased a total of 278,842 shares of our common stock at an average price per share of $27.36, for a total purchase price of $7.6 million, and we completed a “modified Dutch auction” tender offer pursuant to which we purchased 5,532,708 shares of our common stock from our stockholders at a price of $29.25 per share, for a total purchase price of $161.8 million, exclusive of $0.7 million of fees and expenses. At December 31, 2017, we had approximately $129.4 million remaining for the repurchase of our common stock under the October 17, 2016 Board of Directors authorization.

In 2017, 2016 and 2015, we issued 421,108 treasury shares, 363,368 treasury shares and 292,320 treasury shares, respectively, at an average cost of $3.15 per share, for restricted stock units that vested during these years. In 2017, 2016 and 2015, we repurchased 138,080 shares, 134,122 shares and 101,682 shares of our common stock, respectively, at an average cost of $30.24, $25.58 and $28.44 per share, respectively, in accordance with the Plan to satisfy employee withholding tax requirements resulting from certain restricted stock units becoming vested.

We account for treasury shares using the FIFO cost method. As of December 31, 2017, 64,727,152 shares of our common stock were held in treasury.