New Accounting Standards (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income	$ 171,612	$ 172,317	$ 179,706	$ 200,502
Reclassification of income tax effect related to tax reform			(8,552)
APS
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income	182,699	$ 173,982	197,170	$ 202,017
Reclassification of income tax effect related to tax reform			(5,038)
Accounting Standards Update 2017-07
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income	$ 3,000		7,000
Accounting Standards Update 2017-07 | Scenario, Forecast
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net periodic benefit cost (credit)					$ 15,000
Accounting Standards Update 2018-02
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Reclassification of income tax effect related to tax reform			9,000
Accounting Standards Update 2018-02 | APS
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Reclassification of income tax effect related to tax reform			$ 5,000