Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Dec. 31, 2018
Municipal Bonds [Member]
Assets:
Investments in lieu of retainage		$ 1,300	$ 1,200
Money Market Funds [Member]
Assets:
Investments in lieu of retainage		66,900	62,900
US Government Agencies Debt Securities [Member]
Assets:
Restricted and other investments		28,300	27,700
Corporate Debt Securities [Member]
Assets:
Restricted and other investments		35,600	30,400
Fair Value Measured On A Recurring Basis [Member]
Assets:
Cash and cash equivalents	[1]	101,482	116,075
Restricted cash	[1]	5,095	3,788
Restricted investments	[2]	63,937	58,142
Investments in lieu of retainage	[3]	68,187	64,048
Total assets		238,701	242,053
Fair Value Measured On A Recurring Basis [Member] | Quoted Prices In Active Markets (Level 1) [Member]
Assets:
Cash and cash equivalents	[1]	101,482	116,075
Restricted cash	[1]	5,095	3,788
Investments in lieu of retainage	[3]	66,933	62,858
Total assets		173,510	182,721
Fair Value Measured On A Recurring Basis [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets:
Restricted investments	[2]	63,937	58,142
Investments in lieu of retainage	[3]	1,254	1,190
Total assets		$ 65,191	$ 59,332
Maximum [Member]
Assets:
Restricted And other investments, term		5 years

[1] Includes money market funds with original maturity dates of three months or less.

[2] Restricted investments, as of March 31, 2019, consist of investments in corporate debt securities of $35.6 million and U.S. government agency securities of $28.3 million with maturities up to five years, and are valued based on pricing models, which are determined from a compilation of primarily observable market information, broker quotes in non-active markets or similar assets and are therefore classified as Level 2 assets. As of December 31, 2018, restricted investments consisted of investments in corporate debt securities of $30.4 million and U.S. government agency securities of $27.7 million. The amortized cost of these securities at March 31, 2019 and December 31, 2018 was not materially different from the fair value.

[3] Investments in lieu of retainage are included in retainage receivable and as of March 31, 2019 are comprised of money market funds of $66.9 million and municipal bonds of $1.3 million. The fair values of the money market funds are measured using quoted market prices; therefore, they are classified as Level 1 assets. The fair values of municipal bonds are measured using readily available pricing sources for comparable instruments; therefore, they are classified as Level 2 assets. As of December 31, 2018, investments in lieu of retainage consisted of money market funds of $62.9 million and municipal bonds of $1.2 million. The amortized cost of these available-for-sale securities at March 31, 2019 and December 31, 2018 was not materially different from the fair value.