Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended
		Apr. 11, 2018	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018
Equity [Abstract]
Stock Options Excluded From Diluted Common Shares Outstanding Calculation			4,000,000	4,900,000
Shares, Issued			281,614
Common stock dividend (in dollars per share)			$ 0.89	$ 0.87
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			$ 15,505
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

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