Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended			6 Months Ended
		Jul. 27, 2018	Jun. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018
Equity [Abstract]
Stock Options Excluded From Diluted Common Shares Outstanding Calculation				1,100,000		2,500,000	1,100,000	3,700,000
Forward Contract Indexed to Issuer's Equity, Indexed Shares			15,300,000	15,300,000			15,300,000
Forward Contract Indexed to Issuer's Equity, Shares			1,200,000
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share			$ 74.45
Shares, Issued			323,364	323,364			323,364
Common stock dividend (in dollars per share)				$ 0.89		$ 0.87	$ 1.78	$ 1.74
Accumulated Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, after Tax									$ 632,617
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	[1]								$ 56,360
Tax Cuts and Jobs Act, Reclassification from AOCI resulting from stranded tax effects					$ 15,500		$ 15,505
Tax Cuts and Jobs Act, Portion Reclassified from AOCI to Retained Earnings					32,000
Tax Cuts and Jobs Act, Portion Reclassified from AOCI to Regulatory Liability for Income Taxes					$ 16,500
Subsequent Event [Member]
Equity [Abstract]
Common stock dividend (in dollars per share)		$ 0.89

[1] (a) Consolidated net income and preferred dividend requirements of subsidiaries for 2018 and 2017 include $6.9 million and $6.9 million, respectively, of preferred dividends on subsidiaries’ preferred stock without sinking fund that is not presented within equity.