SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - STATEMENTS OF CASH FLOWS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net cash provided by operating activities	$ 512,470	$ 499,312	$ 420,441
Cash flows from investing activities
Capital expenditures	(457,520)	(537,369)	(495,187)
Other, net	13,292	14,534	7,487
Net cash used in investing activities	(541,709)	(792,059)	(815,299)
Cash flows from financing activities
Net increase (decrease) in short-term borrowings with original maturities of three months or less	86,718	(18,999)	67,992
Proceeds from issuance of short-term debt	75,000	25,000	125,000
Repayment of short-term debt	(50,000)	(50,000)	(75,000)
Proceeds from issuance of long-term debt	289,349	250,000	532,325
Repayment of long-term debt and funds transferred for repayment of long-term debt	(287,285)	(53,887)	(465,000)
Common stock dividends	(139,463)	(134,987)	(134,873)
Other	(1,836)	(1,603)	(6,349)
Net cash used in financing activities	87,716	200,074	378,287
Net decrease in cash and equivalents	58,477	(92,673)	(16,571)
Cash, cash equivalents and restricted cash, January 1	169,208	261,881	278,452
Cash, cash equivalents and restricted cash, December 31	227,685	169,208	261,881
Parent Company
Cash flows from operating activities
Net cash provided by operating activities	131,120	135,470	99,600
Cash flows from investing activities
Increase in note receivable from subsidiary	(1,187)	(20,596)	(70,000)
Decrease in note receivable from subsidiary	0	0	66,391
Capital expenditures	(47)	(143)	(317)
Investments in subsidiaries	(38,935)	(71,970)	(22,353)
Other, net	(1,001)	140	(177)
Net cash used in investing activities	(41,170)	(92,569)	(26,456)
Cash flows from financing activities
Net increase (decrease) in notes payable to subsidiaries with original maturities of three months or less	0	(30)	98
Net increase (decrease) in short-term borrowings with original maturities of three months or less	47,731	(14,000)	62,993
Proceeds from issuance of short-term debt	0	0	125,000
Repayment of short-term debt	0	(50,000)	(75,000)
Proceeds from issuance of long-term debt	0	150,000	150,000
Repayment of long-term debt and funds transferred for repayment of long-term debt	0	0	(200,000)
Withheld shares for employee taxes on vested share-based compensation	(997)	(996)	(3,828)
Common stock dividends	(139,463)	(134,987)	(134,873)
Other	(10)	(848)	(756)
Net cash used in financing activities	(92,739)	(50,861)	(76,366)
Net decrease in cash and equivalents	(2,789)	(7,960)	(3,222)
Cash, cash equivalents and restricted cash, January 1	3,742	11,702	14,924
Cash, cash equivalents and restricted cash, December 31	$ 953	$ 3,742	$ 11,702