Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Assets:
Restricted Investments		$ 70,974	$ 58,142
Municipal Bonds [Member]
Assets:
Investments in lieu of retainage		1,200	1,200
Fair Value, Measurements, Recurring [Member]
Assets:
Cash and cash equivalents	[1]	193,685	116,075
Restricted cash	[1]	8,416	3,788
Restricted investments	[2]	70,974	58,142
Investments in lieu of retainage	[3]	90,791	64,048
Total		363,866	242,053
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	[1]	193,685	116,075
Restricted cash	[1]	8,416	3,788
Investments in lieu of retainage	[3]	89,572	62,858
Total		291,673	182,721
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Restricted investments	[2]	70,974	58,142
Investments in lieu of retainage	[3]	1,219	1,190
Total		72,193	$ 59,332
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Cash and cash equivalents	[1]
Restricted cash	[1]
Restricted investments	[2]
Investments in lieu of retainage	[3]
Total
Maximum [Member]
Assets:
Restricted investment maturity period		5 years	5 years
Corporate Debt Securities [Member]
Assets:
Restricted and other investments		$ 35,800	$ 30,400
US Government Agencies Securities [Member]
Assets:
Restricted and other investments		33,800	27,700
Certificates of Deposit [Member]
Assets:
Restricted and other investments		1,400
Money Market Funds [Member]
Assets:
Investments in lieu of retainage		$ 89,600	$ 62,900

[1] Includes money market funds and short-term investments with maturity dates of three months or less when acquired.

[2] Restricted investments, as of December 31, 2019, consist of investments in corporate debt securities of $35.8 million, U.S. government agency securities of $33.8 million and corporate certificates of deposits of $1.4 million with maturities of 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 with maturities of up to five years. The amortized cost of these available-for-sale securities at December 31, 2019 and 2018 was not materially different from the fair value.

[3] Investments in lieu of retainage are included in retainage receivable and as of December 31, 2019 are comprised of money market funds of $89.6 million and municipal bonds of $1.2 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 December 31, 2019 and 2018 was not materially different from the fair value. The Company did not have material transfers between Levels 1 and 2 during the years ended December 31, 2019 and 2018.