Investments (Available-For-Sale Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 3,813,857	$ 3,758,720
Total Fair Value	3,544,137	3,493,597
U.S. Treasury obligations
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	258,462	259,834
Due in one year or less	36,249
Due after one year through five years	169,651
Due after five years through ten years	32,387
Due after ten years	2,602
Total Fair Value	240,889	243,525
U.S. Government-sponsored enterprise obligations
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	19,957	19,752
Due in one year or less	4,790
Due after one year through five years	11,653
Due after five years through ten years	505
Due after ten years	1,979
Total Fair Value	18,927	18,724
State and municipal bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	502,549	482,367
Due in one year or less	30,409
Due after one year through five years	157,707
Due after five years through ten years	161,274
Due after ten years	124,875
Total Fair Value	474,265	454,381
Corporate debt
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1,849,162	1,883,308
Due in one year or less	179,206
Due after one year through five years	886,529
Due after five years through ten years	569,033
Due after ten years	79,284
Total Fair Value	1,714,052	1,750,574
Residential mortgage-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	507,312	481,267
Total Fair Value	450,052	430,137
Agency commercial mortgage-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	8,964	9,369
Total Fair Value	7,950	8,387
Other commercial mortgage-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	248,756	210,469
Total Fair Value	229,029	189,474
Other asset-backed securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	418,695	412,354
Total Fair Value	$ 408,973	$ 398,395