Employee Benefits Plans (Details 6) - Pension Plans - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,941,330	$ 1,787,075	$ 1,647,286
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,826,737	1,684,191
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	114,593	102,884	$ 100,631
Private equity and hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	114,593	102,884
Private equity and hedge funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private equity and hedge funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private equity and hedge funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	114,593	102,884
Equity securities: | U.S. common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	443,405	429,456
Equity securities: | U.S. common collective trusts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Equity securities: | U.S. common collective trusts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	443,405	429,456
Equity securities: | U.S. common collective trusts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Equity securities: | Foreign common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	458,111	398,282
Equity securities: | Foreign common collective trusts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Equity securities: | Foreign common collective trusts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	458,111	398,282
Equity securities: | Foreign common collective trusts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income securities: | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	85,117	76,086
Fixed income securities: | Corporate bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income securities: | Corporate bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	85,117	76,086
Fixed income securities: | Corporate bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income securities: | Common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	840,104	780,367
Fixed income securities: | Common collective trusts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income securities: | Common collective trusts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	840,104	780,367
Fixed income securities: | Common collective trusts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0