Employee Benefits Plans (Details 6) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 102,884	$ 100,631	$ 89,727
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,787,075	1,647,286	$ 1,775,417
Pension Plans | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,684,191	1,546,655
Pension Plans | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	102,884	100,631
Pension Plans | Private equity and hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	102,884	100,631
Pension Plans | Private equity and hedge funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans | Private equity and hedge funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans | Private equity and hedge funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	102,884	100,631
Pension Plans | Equity securities: | U.S. common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	429,456	387,123
Pension Plans | Equity securities: | U.S. common collective trusts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans | Equity securities: | U.S. common collective trusts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	429,456	387,123
Pension Plans | Equity securities: | U.S. common collective trusts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans | Equity securities: | Foreign common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	398,282	374,858
Pension Plans | Equity securities: | Foreign common collective trusts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans | Equity securities: | Foreign common collective trusts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	398,282	374,858
Pension Plans | Equity securities: | Foreign common collective trusts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans | Fixed income securities: | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	76,086	64,834
Pension Plans | Fixed income securities: | Corporate bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans | Fixed income securities: | Corporate bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	76,086	64,834
Pension Plans | Fixed income securities: | Corporate bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans | Fixed income securities: | Common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	780,367	719,840
Pension Plans | Fixed income securities: | Common collective trusts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Plans | Fixed income securities: | Common collective trusts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	780,367	719,840
Pension Plans | Fixed income securities: | Common collective trusts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0