Composition of Certain Financial Statement Captions (Deferred Contract Costs) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Items Included in Consolidated Statement of Financial Condition [Abstract]
Deferred contract costs	$ 33,352,000		$ 33,352,000		$ 33,781,000
Capitalized contract cost, amount capitalized during period	$ 1,674,000	$ 1,752,000	4,046,000	$ 4,749,000
Amortization of deferred contract costs			4,475,000	$ 2,632,000
Capitalized contract cost impairment			$ 0