Fair Value Measurements (Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Assets:
Restricted cash, primarily money market funds	$ 30,376		$ 12,793
Liabilities:
Gain included in interest and other income/expense	(6,000)	$ 3,500	9,400
Equity securities, unrealized gain (loss)	(6,300)	$ 3,000	6,700
Recurring [Member]
Assets:
Assets, fair value disclosure	514,956		76,624
Liabilities:
Deferred compensation plan obligations	32,358		38,378
Total	32,358		38,378
Recurring [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	473,502		28,330
Restricted cash, primarily money market funds	9,096		9,916
Recurring [Member] | Grantor Trusts [Member]
Assets:
Investments	32,358		38,378
Recurring [Member] | Quoted Prices In Active Markets For Identical Assets And Liabilities (Level 1) [Member]
Assets:
Assets, fair value disclosure	514,956		76,624
Liabilities:
Deferred compensation plan obligations	32,358		38,378
Total	32,358		38,378
Recurring [Member] | Quoted Prices In Active Markets For Identical Assets And Liabilities (Level 1) [Member] | Money Market Funds [Member]
Assets:
Cash equivalents	473,502		28,330
Restricted cash, primarily money market funds	9,096		9,916
Recurring [Member] | Quoted Prices In Active Markets For Identical Assets And Liabilities (Level 1) [Member] | Grantor Trusts [Member]
Assets:
Investments	32,358		38,378
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets:
Assets, fair value disclosure	0		0
Liabilities:
Total	0		0
Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets:
Assets, fair value disclosure	0		0
Liabilities:
Total	$ 0		$ 0