Postretirement Benefits (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Postretirement benefits
Decrease in projected benefit obligation	$ (12,100)
Funded status at end of year	(173,089)	$ (188,917)
Net amount recognized in the Statements of Financial Position:
Pension liabilities - due within one year	(389)	(97)
Pension liabilities - due after one year	(172,700)	(188,820)
Net amount recognized	$ (173,089)	$ (188,917)
Employee pension plan
Postretirement benefits
Funding target (as a percentage of the funding target liability)	100.00%
Expected employer contribution during next fiscal year	$ 17,400