New Accounting Standards (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 5,115,071		$ 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	1,600
Accounting Standards Update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from AOCI to retained earnings			252,000
Scenario, Forecast [Member] | Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit costs		$ 3,500
Fair Value Option [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investments, fair value disclosure	108,452
Partnership Interest [Member] | Fair Value Option [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investments, fair value disclosure	$ 108,452		$ 0