Fair Value of Financial Instruments (Quantitative Information About Level 3 Fair Value Measurements) (Details) $ in Thousands	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	$ 1,816,765	$ 1,825,438
Nonredeemable preferred stocks	193,263	206,685
Recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	1,816,939	1,825,443
Nonredeemable preferred stocks	193,263	206,685
Recurring | Asset-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	848	926
Recurring | Nonredeemable preferred stocks
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Nonredeemable preferred stocks	595	595
Recurring | Financial services
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Nonredeemable preferred stocks	53,228	69,577
Recurring | Financial services | Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	106,019	101,031
Recurring | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	998	1,176
Nonredeemable preferred stocks	595	595
Recurring | Level 3 | Asset-backed securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	$ 848	926
Recurring | Level 3 | Asset-backed securities | Discounted cash flow | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, range of weighted average significant unobservable inputs	0.04
Recurring | Level 3 | Asset-backed securities | Discounted cash flow | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities, range of weighted average significant unobservable inputs	0.06
Recurring | Level 3 | Nonredeemable preferred stocks
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Nonredeemable preferred stocks	$ 595	595
Recurring | Level 3 | Nonredeemable preferred stocks | Discounted cash flow | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Nonredeemable preferred stocks, range of weighted average significant unobservable inputs	0.03
Recurring | Level 3 | Nonredeemable preferred stocks | Discounted cash flow | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Nonredeemable preferred stocks, range of weighted average significant unobservable inputs	0.04
Recurring | Level 3 | Financial services
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Nonredeemable preferred stocks	$ 0	0
Recurring | Level 3 | Financial services | Corporate bonds
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Available-for-sale securities	$ 150	$ 250