Pension Plans and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Qualified Defined Contribution Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	$ 0	$ 0	$ 4,580
Total assets at the end of the year	0	9,118	123,008
401 K Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	6,918	6,466	2,753
Total assets at the end of the year	177,352	161,956	132,053
Non Qualified Defined Contribution Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions to employees' accounts	72	122	74
Total assets at the end of the year	$ 0	$ 0	$ 0