New Accounting Standards (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 5,267,964		$ 4,806,208
Accounting Standards Update 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings				$ 4,900
Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit costs	2,400
Accounting Standards Update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from AOCI to retained earnings			252,000
Forecast [Member] | Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit costs		$ 3,200
Fair Value Option [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investments, fair value disclosure	107,996
Partnership Interest [Member] | Fair Value Option [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investments, fair value disclosure	$ 107,996		$ 0