Pension Plan and Postretirement Benefits Other Than Pension (Details 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Weighted average assumptions used to determine net periodic benefit cost
Expected return on plan assets (as a percent)	7.75%
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 12,080	$ 10,084	$ 11,011
Interest cost	8,420	8,395	7,711
Expected return on plan assets	(14,510)	(14,016)	(11,185)
Actuarial (gain) loss amortization	5,171	1,496	4,567
Prior service cost amortization	459	468	555
Transition obligation amortization	5	5	5
Net periodic benefit cost	11,625	$ 6,432	$ 12,664
Amounts that will be amortized from accumulated other comprehensive income into net periodic benefit cost in next fiscal year
Estimated net gain (loss) that will be amortized from accumulated other comprehensive income into net periodic benefit cost	(6,100)
Estimated prior service cost that will be amortized from accumulated other comprehensive income into net periodic benefit cost	374
Estimated transition obligation that will be amortized from accumulated other comprehensive income into net periodic benefit cost	$ 5
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	4.13%	4.97%	4.22%
Expected return on plan assets (as a percent)	7.75%	7.75%	7.75%
Rate of compensation increase (as a percent)	5.12%	5.12%	3.99%
Expected benefit payments
2016	$ 7,654
2017	8,274
2018	9,557
2019	10,518
2020	13,115
2021 through 2025	78,851
Total	127,969
Expected contribution in the next fiscal year	20,000
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	910	$ 719	$ 788
Interest cost	397	397	361
Actuarial (gain) loss amortization		(17)
Prior service cost amortization	19	55	55
Net periodic benefit cost	1,326	$ 1,154	$ 1,204
Amounts that will be amortized from accumulated other comprehensive income into net periodic benefit cost in next fiscal year
Estimated net gain (loss) that will be amortized from accumulated other comprehensive income into net periodic benefit cost	153
Estimated prior service cost that will be amortized from accumulated other comprehensive income into net periodic benefit cost	$ 4
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	4.07%	4.94%	4.18%
Expected benefit payments
2016	$ 316
2017	343
2018	397
2019	454
2020	487
2021 through 2025	3,253
Total	5,250
Contributions by participants	$ 349	$ 381	$ 321