Retirement Plans & Other Postretirement Benefits - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) plan pension_plan union	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of qualified defined benefit pension plans | pension_plan	3
Portion of benefit costs capitalized as Utility plant	$ 3,000,000	$ 1,900,000	$ 3,100,000
Fair Value of guaranteed annuity contract [Roll Forward]
Guaranteed Annuity Contract - estimate of undiscounted funds necessary to satisfy John Hancock's remaining obligation	$ 4,200,000	$ 4,000,000
Guaranteed Annuity Contract - percent of composite investment return, net of manger fees and other charges	3.25%	3.60%
Multiemployer Plans [Abstract]
Multiemployer contributions	$ 42,100,000	$ 35,000,000	32,700,000
Approximate number of unions the Company contributed to on behalf of employees | union	250
Average contribution	$ 200,000
Largest contribution	4,800,000
Defined Contribution Plan [Abstract]
Contributions to defined contribution plan	$ 13,200,000	$ 12,100,000	$ 11,000,000
Multiemployer Benefit Plans
Multiemployer Plans [Abstract]
Number of multiemployer plans contributed to | plan	50
Defined benefit plan, number of significant plans | plan	6
Number of plans in red zone status receiving company contributions | pension_plan	2
Number of plans whereby company contributions exceed 5% of plan balance | pension_plan	5
Percentage threshold by which plan contributions are considered significant	5.00%
Number of plans in red zone status considered significant by the Company | pension_plan	1
Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Maximum number of days restriction for exchange of shares in Common CTF's	31 days
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	4.07%	4.31%	4.05%
Discount rate	3.61%	4.07%
Expected return on plan assets	7.00%
Fair Value of guaranteed annuity contract [Roll Forward]
Expected contributions in fiscal year 2018	$ 3,500,000
Estimated future benefit payments [Abstract]
Expected future benefit payments, 2018	28,800,000
Expected future benefit payments, 2019	43,000,000
Expected future benefit payments, 2020	30,100,000
Expected future benefit payments, 2021	27,300,000
Expected future benefit payments, 2022	28,600,000
Expected future benefit payments, 2022-2026	132,700,000
Amounts that will be amortized from Accumulated Other Comprehensive Income (Loss) in next year [Abstract]
Prior service cost expected to be amortized in next year	500,000
Actuarial gain/loss expected to be amortized in next year	8,500,000
SERP
Fair Value of guaranteed annuity contract [Roll Forward]
Payments expected to be made to SERP participants in the next fiscal year	$ 1,100,000
Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	4.04%	4.21%	3.95%
Discount rate	3.57%	4.04%
Fair Value of guaranteed annuity contract [Roll Forward]
Other postretirement benefit payments to be made during the next fiscal year	$ 2,900,000
Estimated future benefit payments [Abstract]
Expected future benefit payments, 2018	4,100,000
Expected future benefit payments, 2019	4,400,000
Expected future benefit payments, 2020	4,700,000
Expected future benefit payments, 2021	4,900,000
Expected future benefit payments, 2022	4,900,000
Expected future benefit payments, 2022-2026	23,100,000
Amounts that will be amortized from Accumulated Other Comprehensive Income (Loss) in next year [Abstract]
Prior service cost expected to be amortized in next year	2,200,000
Actuarial gain/loss expected to be amortized in next year	$ 0