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Summary of significant accounting policies (Tables) | 12 Months Ended
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Dec. 31, 2018
Accounting Policies [Abstract] |
Contract receivable retainage | Amounts representing balances billed but not paid by customers under retainage provisions in contracts at December 31 were as follows:
| 2018 |  | 2017 |
| (In thousands)
Short-term retainage* | $ | 56,228 |  | $ | 57,134 |
Long-term retainage** | 4,152 |  | 1,410 |
Total retainage | $ | 60,380 |  | $ | 58,544 |
* | Expected to be paid within one year or less and included in receivables, net.
** Included in deferred charges and other assets - other.
Inventories | Inventories at December 31 consisted of:
| 2018 |  | 2017 |
| (In thousands)
Aggregates held for resale | $ | 139,681 |  | $ | 115,268 |
Asphalt oil | 54,741 |  | 30,360 |
Materials and supplies | 23,611 |  | 18,650 |
Merchandise for resale | 22,552 |  | 14,905 |
Natural gas in storage (current) | 22,117 |  | 20,950 |
Other | 24,607 |  | 26,450 |
Total | $ | 287,309 |  | $ | 226,583 |
AFUDC and interest capitailized | The amount of AFUDC for the years ended December 31 were as follows:
| 2018 |  | 2017 |  | 2016 |
| (In thousands)
AFUDC - borrowed | $ | 2,290 |  | $ | 966 |  | $ | 914 |
AFUDC - equity | $ | 1,897 |  | $ | 909 |  | $ | 565 |
Property, plant and equipment | Property, plant and equipment at December 31 was as follows:
| 2018 |  | 2017 |  | Weighted Average Depreciable Life in Years |
| (Dollars in thousands, where applicable)
Regulated: |  |  |
Electric: |  |  |
Generation | $ | 1,131,484 |  | $ | 1,034,765 |  | 49 |
Distribution | 430,750 |  | 415,543 |  | 46 |
Transmission | 302,315 |  | 296,941 |  | 64 |
Construction in progress | 161,893 |  | 117,906 |  | — |
Other | 122,127 |  | 117,109 |  | 13 |
Natural gas distribution: |  |  |
Distribution | 1,981,356 |  | 1,831,795 |  | 47 |
Construction in progress | 21,028 |  | 19,823 |  | — |
Other | 496,708 |  | 468,227 |  | 16 |
Pipeline and midstream: |  |  |
Transmission | 585,594 |  | 516,932 |  | 54 |
Gathering | 37,829 |  | 37,837 |  | 20 |
Storage | 49,101 |  | 45,629 |  | 61 |
Construction in progress | 5,915 |  | 17,488 |  | — |
Other | 45,763 |  | 41,054 |  | 33 |
Nonregulated: |  |  |
Pipeline and midstream: |  |  |
Gathering and processing | 31,094 |  | 31,678 |  | 19 |
Construction in progress | 86 |  | 17 |  | — |
Other | 9,577 |  | 9,649 |  | 10 |
Construction materials and contracting: |  |  |
Land | 109,541 |  | 95,745 |  | — |
Buildings and improvements | 114,905 |  | 102,435 |  | 20 |
Machinery, vehicles and equipment | 1,090,790 |  | 947,979 |  | 12 |
Construction in progress | 22,507 |  | 7,750 |  | — |
Aggregate reserves | 430,263 |  | 406,139 |  | * |
Construction services: |  |  |
Land | 5,216 |  | 5,216 |  | — |
Buildings and improvements | 29,795 |  | 27,351 |  | 25 |
Machinery, vehicles and equipment | 145,859 |  | 137,924 |  | 6 |
Other | 7,716 |  | 6,774 |  | 3 |
Other: |  |  |
Land | 2,648 |  | 2,837 |  | — |
Other | 25,461 |  | 28,286 |  | 14 |
Less accumulated depreciation, depletion and amortization | 2,818,644 |  | 2,691,641 |  |
Net property, plant and equipment | $ | 4,578,677 |  | $ | 4,079,188 |  |
* | Depleted on the units-of-production method based on recoverable aggregate reserves.
Schedule of earnings per share reconciliation | A reconciliation of the weighted average common shares outstanding used in the basic and diluted earnings per share calculation was as follows:
| 2018 |  | 2017 |  | 2016 |
| (In thousands)
Weighted average common shares outstanding - basic | 195,720 |  | 195,304 |  | 195,299 |
Effect of dilutive performance share awards | 430 |  | 383 |  | 319 |
Weighted average common shares outstanding - diluted | 196,150 |  | 195,687 |  | 195,618 |
Shares excluded from the calculation of diluted earnings per share | 10 |  | — |  | — |
Schedule of New Accounting Pronouncements and Changes in Accounting Principles | The cumulative effect of the changes made to the Consolidated Balance Sheet were as follows:
| December 31, 2017 |  | Adjustments |  | January 1, 2018 |
| (In thousands)
Liabilities and Stockholders' Equity |  |  |
Current liabilities: |  |  |
Other accrued liabilities | $ | 186,010 |  | $ | 903 |  | $ | 186,913 |
Deferred credits and other liabilities: |  |  |
Deferred income taxes | 347,271 |  | (332 | ) | 346,939 |
Other | 1,179,140 |  | 399 |  | 1,179,539 |
Commitments and contingencies |  |  |
Stockholders' equity: |  |  |
Common stockholders' equity: |  |  |
Retained earnings | 1,040,748 |  | (970 | ) | 1,039,778 |
After-tax changes in the components of accumulated other comprehensive income (loss) | The after-tax changes in the components of accumulated other comprehensive loss as of December 31, 2018, 2017 and 2016, were as follows:
| Net Unrealized Gain (Loss) on Derivative Instruments Qualifying as Hedges |  | Post- retirement Liability Adjustment |  | Foreign Currency Translation Adjustment |  | Net Unrealized Gain (Loss) on Available- for-sale Investments |  | Total Accumulated Other Comprehensive Loss |
| (In thousands)
Balance at December 31, 2016 | $ | (2,300 | ) | $ | (33,221 | ) | $ | (149 | ) | $ | (63 | ) | $ | (35,733 | )
Other comprehensive loss before reclassifications | — |  | (1,812 | ) | (6 | ) | (139 | ) | (1,957 | )
Amounts reclassified from accumulated other comprehensive loss | 366 |  | 1,013 |  | — |  | 120 |  | 1,499 |
Amounts reclassified to accumulated other comprehensive loss from a regulatory asset | — |  | (1,143 | ) | — |  | — |  | (1,143 | )
Net current-period other comprehensive income (loss) | 366 |  | (1,942 | ) | (6 | ) | (19 | ) | (1,601 | )
Balance at December 31, 2017 | (1,934 | ) | (35,163 | ) | (155 | ) | (82 | ) | (37,334 | )
Other comprehensive income (loss) before reclassifications | — |  | 4,441 |  | (61 | ) | (144 | ) | 4,236 |
Amounts reclassified from accumulated other comprehensive loss | 162 |  | 2,173 |  | 249 |  | 131 |  | 2,715 |
Net current-period other comprehensive income (loss) | 162 |  | 6,614 |  | 188 |  | (13 | ) | 6,951 |
Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss | (389 | ) | (7,520 | ) | (33 | ) | (17 | ) | (7,959 | )
Balance at December 31, 2018 | $ | (2,161 | ) | $ | (36,069 | ) | $ | — |  | $ | (112 | ) | $ | (38,342 | )
Reclassification out of accumulated other comprehensive income (loss) | The following amounts were reclassified out of accumulated other comprehensive loss into net income. The amounts presented in parenthesis indicate a decrease to net income on the Consolidated Statements of Income. The reclassifications for the years ended December 31 were as follows:
| 2018 |  | 2017 |  | Location on Consolidated Statements of Income
| (In thousands) |
Reclassification adjustment for loss on derivative instruments included in net income | $ | (591 | ) | $ | (590 | ) | Interest expense
| 429 |  | 224 |  | Income taxes
| (162 | ) | (366 | ) |
Amortization of postretirement liability losses included in net periodic benefit cost (credit) | (2,894 | ) | (1,658 | ) | Other income
| 721 |  | 645 |  | Income taxes
| (2,173 | ) | (1,013 | ) |
Reclassification adjustment for loss on foreign currency translation adjustment included in net income | (324 | ) | — |  | Other income
| 75 |  | — |  | Income taxes
| (249 | ) | — |  |
Reclassification adjustment for loss on available-for-sale investments included in net income | (166 | ) | (185 | ) | Other income
| 35 |  | 65 |  | Income taxes
| (131 | ) | (120 | ) |
Total reclassifications | $ | (2,715 | ) | $ | (1,499 | ) |

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