Marketable Securities - Fair Value of and Gross Unrealized Losses on Investment Securities (Details) $ in Thousands	Mar. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($)
Marketable securities
Less than 12 months - Fair value	$ 46,190	$ 173,112
Less than 12 months - Unrealized loss	(847)	(858)
12 months or more - Fair value	11,487	161,944
12 months or more - Unrealized loss	(200)	(2,087)
Total - Fair value	57,677	335,056
Total - Unrealized loss	$ (1,047)	(2,945)
Number of available-for-sale debt securities in unrealized loss position | security	83
U.S. government and agencies
Marketable securities
Less than 12 months - Fair value	$ 0	0
Less than 12 months - Unrealized loss	0	0
12 months or more - Fair value	2,320	52,620
12 months or more - Unrealized loss	(103)	(336)
Total - Fair value	2,320	52,620
Total - Unrealized loss	(103)	(336)
Residential mortgage-backed securities | Agency CMOs
Marketable securities
Less than 12 months - Fair value	46,190	173,112
Less than 12 months - Unrealized loss	(847)	(858)
12 months or more - Fair value	9,167	109,324
12 months or more - Unrealized loss	(97)	(1,751)
Total - Fair value	55,357	282,436
Total - Unrealized loss	$ (944)	$ (2,609)