SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - BALANCE SHEETS (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Cash and cash equivalents	$ 196,813	$ 169,208
Accounts receivable	300,794	325,672
Property, plant and equipment, net	5,109,628	4,830,118
Other assets and intercompany receivables	649,885	530,364
Total assets	13,745,251	13,104,051	$ 12,534,160
Liabilities
Accounts payable	220,633	214,773
Long-term debt, net	1,964,365	1,879,641
Other	583,545	580,788
Total liabilities	11,430,698	10,907,478
Shareholders' equity
Preferred stock, no par value, authorized 10,000,000 shares; issued: none	0	0
Common stock equity	1,678,257	1,669,267
Retained earnings	622,042	543,623
Accumulated other comprehensive loss	(20,039)	(50,610)
Total shareholders’ equity	2,280,260	2,162,280	$ 2,097,386	$ 2,066,753
Total capitalization and liabilities	$ 13,745,251	$ 13,104,051
Preferred stock, authorized shares	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Common stock, authorized shares	200,000,000	200,000,000
Common stock, issued shares	108,973,328	108,879,245
Common stock, outstanding shares	108,973,328	108,879,245
Parent Company
Assets
Cash and cash equivalents	$ 953	$ 3,742
Accounts receivable	779	2,604
Notes receivable from subsidiaries	22,598	20,789
Property, plant and equipment, net	2,931	3,456
Deferred income tax assets	10,754	10,147
Other assets and intercompany receivables	21,770	11,963
Investments in subsidiaries, at equity	2,761,802	2,605,038
Total assets	2,821,587	2,657,739
Liabilities
Accounts payable	1,509	2,001
Interest payable	3,041	3,476
Notes payable to subsidiaries	0	34
Commercial paper	96,723	48,992
Long-term debt, net	399,064	398,874
Retirement benefits liability	29,367	29,565
Other	11,623	12,517
Total liabilities	541,327	495,459
Shareholders' equity
Preferred stock, no par value, authorized 10,000,000 shares; issued: none	0	0
Common stock equity	1,678,257	1,669,267
Retained earnings	622,042	543,623
Accumulated other comprehensive loss	(20,039)	(50,610)
Total shareholders’ equity	2,280,260	2,162,280
Total capitalization and liabilities	$ 2,821,587	$ 2,657,739
Preferred stock, authorized shares	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Common stock, authorized shares	200,000,000	200,000,000
Common stock, issued shares	108,973,328	108,879,245
Common stock, outstanding shares	108,973,328	108,879,245