Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Assets:
Restricted cash		$ 3,788	$ 4,780
Restricted Investments At Amortized Cost		60,100
Restricted Investments		58,142	53,014
Fair Value, Measurements, Recurring [Member]
Assets:
Cash and cash equivalents	[1]	116,075	192,868
Restricted cash	[1]	3,788	4,780
Restricted investments	[2]	58,142
Investments in lieu of retainage	[3]	64,048	72,296
Total		242,053	269,944
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets (Level 1) [Member]
Assets:
Cash and cash equivalents	[1]	116,075	192,868
Restricted cash	[1]	3,788	4,780
Investments in lieu of retainage	[3]	62,858	69,891
Total		182,721	267,539
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Restricted investments	[2]	58,142
Investments in lieu of retainage	[3]	1,190	2,405
Total		59,332	2,405
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
Corporate Debt Securities [Member]
Assets:
Restricted Investments		30,400	33,000
US Government Agencies Securities [Member]
Assets:
Restricted Investments		$ 27,700	$ 26,100

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

[2] During 2018, the Company reclassified its restricted investments from the held-to-maturity category to the available-for-sale category as a result of a change in management's investment strategy. At the time of the transfer, the securities had an aggregate amortized cost of $60.1 million and an immaterial aggregate unrealized loss. Restricted investments, as of December 31, 2018, consist 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, 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, 2017, restricted investments consisted of investments in U.S. agency securities of $26.1 million and corporate debt securities of $33.0 million. The amortized cost of these securities at December 31, 2018 and 2017 was not materially different from the fair value.

[3] Investments in lieu of retainage are included in retainage receivable and as of December 31, 2018 are comprised of money market funds of $62.9 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, 2017, investments in lieu of retainage consisted of money market funds of $69.9 million and municipal bonds of $2.4 million. The amortized cost of these available-for-sale securities at December 31, 2018 and 2017 was not materially different from the fair value.