New Accounting Standards (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		$ 4,955,608		$ 4,806,208
Impact of ASU on financial statements	[1]			(4,896)
Accounting Standards Update 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings					$ 4,900
Accounting Standards Update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from AOCI to retained earnings				252,000
Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit costs		831
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		106,151
Partnership Interest [Member] | Fair Value Option [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investments, fair value disclosure		$ 106,000		$ 0

[1] See further discussion in Note 2—New Accounting Standards and Note 4—Investments.