Carrying Amount and Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value Disclosures [Abstract]
Cash and cash equivalents, Carrying value	$ 98,758	$ 164,188	$ 35,972	$ 50,007
Insurance subsidiary investments, Carrying value	311,136	265,996
Long-term debt, including amounts due within one year, carrying value	3,160,807	2,877,138
Cash and cash equivalents, Fair value	98,758	164,188
Insurance subsidiary investments, Fair value	311,136	265,996
Long-term debt, including amounts due within one year, Fair value	$ 2,978,890	$ 2,930,815