Pension Plans and Other Postretirement Benefits (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Qualified Defined Contribution Plan [Member]
Summary of contributions to qualified defined contribution plan, 401(k), non-qualified defined contribution plan and total assets of the plans
Contributions to employees' accounts	$ 4,580	$ 4,616	$ 4,148
Total assets at the end of the year	117,720	135,097	141,286
401 K Plan [Member]
Summary of contributions to qualified defined contribution plan, 401(k), non-qualified defined contribution plan and total assets of the plans
Contributions to employees' accounts	2,753	2,781	2,753
Total assets at the end of the year	132,053	134,897	118,073
Non Qualified Defined Contribution Plan [Member]
Summary of contributions to qualified defined contribution plan, 401(k), non-qualified defined contribution plan and total assets of the plans
Contributions to employees' accounts	74	110	0
Total assets at the end of the year	$ 0	$ 0	$ 0