Postretirement Benefits (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Postretirement benefits
Increase in projected benefit obligations	$ 92,100
Number of additional years of updated mortality data that contributed to the increase in projected benefit obligations (in years)	2 years
Funded status at end of year	$ (224,115)	$ (173,089)
Net amount recognized in the Statements of Financial Position:
Pension liabilities - due within one year	(2,288)	(389)
Pension liabilities - due after one year	(221,827)	(172,700)
Net amount recognized	$ (224,115)	$ (173,089)
Employee pension plan
Postretirement benefits
Funding target (as a percentage of the funding target liability)	100.00%
Expected employer contribution during next fiscal year	$ 19,000