Fair Value Measurements - Information about Financial Assets Measured at Fair Value on Recurring Basis Based on Three-Tier Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Level 3 | Redeemable Preferred Stock Investment
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	$ 20,000
Fair Value Measurements Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total equity securities, debt securities and cash equivalents	732,811	$ 391,904
Fair Value Measurements Recurring | Bond Fund
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total equity trading securities	128,987	153,185
Fair Value Measurements Recurring | Exchange Traded Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total equity trading securities	35,090	17,609
Fair Value Measurements Recurring | Redeemable Preferred Stock Investment
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	20,000
Fair Value Measurements Recurring | Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	357,762	166,599
Fair Value Measurements Recurring | Money Market Accounts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents fair value disclosure	50,425	47,461
Fair Value Measurements Recurring | Municipal Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	24,389
Fair Value Measurements Recurring | U.S. Agency Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	39,693	1,002
Fair Value Measurements Recurring | Yankee Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	12,330	6,048
Fair Value Measurements Recurring | U.S. Government Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	64,135
Fair Value Measurements Recurring | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total equity securities, debt securities and cash equivalents	214,502	218,255
Fair Value Measurements Recurring | Level 1 | Bond Fund
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total equity trading securities	128,987	153,185
Fair Value Measurements Recurring | Level 1 | Exchange Traded Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total equity trading securities	35,090	17,609
Fair Value Measurements Recurring | Level 1 | Redeemable Preferred Stock Investment
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	0
Fair Value Measurements Recurring | Level 1 | Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	0	0
Fair Value Measurements Recurring | Level 1 | Money Market Accounts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents fair value disclosure	50,425	47,461
Fair Value Measurements Recurring | Level 1 | Municipal Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	0
Fair Value Measurements Recurring | Level 1 | U.S. Agency Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	0	0
Fair Value Measurements Recurring | Level 1 | Yankee Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	0	0
Fair Value Measurements Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total equity securities, debt securities and cash equivalents	498,309	173,649
Fair Value Measurements Recurring | Level 2 | Bond Fund
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total equity trading securities	0	0
Fair Value Measurements Recurring | Level 2 | Exchange Traded Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total equity trading securities	0	0
Fair Value Measurements Recurring | Level 2 | Redeemable Preferred Stock Investment
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	0
Fair Value Measurements Recurring | Level 2 | Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	357,762	166,599
Fair Value Measurements Recurring | Level 2 | Money Market Accounts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents fair value disclosure	0	0
Fair Value Measurements Recurring | Level 2 | Municipal Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	24,389
Fair Value Measurements Recurring | Level 2 | U.S. Agency Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	39,693	1,002
Fair Value Measurements Recurring | Level 2 | Yankee Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	12,330	6,048
Fair Value Measurements Recurring | Level 2 | U.S. Government Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	64,135
Fair Value Measurements Recurring | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total equity securities, debt securities and cash equivalents	20,000	0
Fair Value Measurements Recurring | Level 3 | Bond Fund
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total equity trading securities	0	0
Fair Value Measurements Recurring | Level 3 | Exchange Traded Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total equity trading securities	0	0
Fair Value Measurements Recurring | Level 3 | Redeemable Preferred Stock Investment
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	20,000
Fair Value Measurements Recurring | Level 3 | Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	0	0
Fair Value Measurements Recurring | Level 3 | Money Market Accounts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents fair value disclosure	0	0
Fair Value Measurements Recurring | Level 3 | Municipal Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	0
Fair Value Measurements Recurring | Level 3 | U.S. Agency Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	0	0
Fair Value Measurements Recurring | Level 3 | Yankee Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total debt securities	$ 0	$ 0