Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Total fixed maturity securities available for sale	$ 13,654,253	$ 13,528,177
Equity securities	416,588	279,006
Arbitrage trading account	533,442	452,548
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	13,654,253	13,528,177
Equity securities	416,588	279,006
Arbitrage trading account	533,442	452,548
Assets, fair value disclosure	14,604,283	14,259,731
Liabilities:
Securities sold but not yet purchased	159,349	38,120
Fair Value, Measurements, Recurring | U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	777,070	702,240
Fair Value, Measurements, Recurring | State and municipal
Assets:
Total fixed maturity securities available for sale	3,747,823	3,872,598
Fair Value, Measurements, Recurring | Mortgage-backed securities
Assets:
Total fixed maturity securities available for sale	1,575,866	1,594,546
Fair Value, Measurements, Recurring | Asset-backed
Assets:
Total fixed maturity securities available for sale	2,701,785	2,438,747
Fair Value, Measurements, Recurring | Corporate
Assets:
Total fixed maturity securities available for sale	4,036,285	4,111,311
Fair Value, Measurements, Recurring | Foreign government
Assets:
Total fixed maturity securities available for sale	815,424	808,735
Fair Value, Measurements, Recurring | Common stocks
Assets:
Equity securities	140,226	98,192
Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	276,362	180,814
Level 1 | Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	131,242	89,596
Arbitrage trading account	424,030	353,335
Assets, fair value disclosure	555,272	442,931
Liabilities:
Securities sold but not yet purchased	144,420	37,327
Level 1 | Fair Value, Measurements, Recurring | U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	0	0
Level 1 | Fair Value, Measurements, Recurring | State and municipal
Assets:
Total fixed maturity securities available for sale	0	0
Level 1 | Fair Value, Measurements, Recurring | Mortgage-backed securities
Assets:
Total fixed maturity securities available for sale	0	0
Level 1 | Fair Value, Measurements, Recurring | Asset-backed
Assets:
Total fixed maturity securities available for sale	0	0
Level 1 | Fair Value, Measurements, Recurring | Corporate
Assets:
Total fixed maturity securities available for sale	0	0
Level 1 | Fair Value, Measurements, Recurring | Foreign government
Assets:
Total fixed maturity securities available for sale	0	0
Level 1 | Fair Value, Measurements, Recurring | Common stocks
Assets:
Equity securities	131,242	89,596
Level 1 | Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	0	0
Level 2 | Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	13,654,228	13,528,078
Equity securities	272,426	176,869
Arbitrage trading account	83,206	81,905
Assets, fair value disclosure	14,009,860	13,786,852
Liabilities:
Securities sold but not yet purchased	213	0
Level 2 | Fair Value, Measurements, Recurring | U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	777,070	702,240
Level 2 | Fair Value, Measurements, Recurring | State and municipal
Assets:
Total fixed maturity securities available for sale	3,747,823	3,872,598
Level 2 | Fair Value, Measurements, Recurring | Mortgage-backed securities
Assets:
Total fixed maturity securities available for sale	1,575,866	1,594,546
Level 2 | Fair Value, Measurements, Recurring | Asset-backed
Assets:
Total fixed maturity securities available for sale	2,701,760	2,438,648
Level 2 | Fair Value, Measurements, Recurring | Corporate
Assets:
Total fixed maturity securities available for sale	4,036,285	4,111,311
Level 2 | Fair Value, Measurements, Recurring | Foreign government
Assets:
Total fixed maturity securities available for sale	815,424	808,735
Level 2 | Fair Value, Measurements, Recurring | Common stocks
Assets:
Equity securities	0	0
Level 2 | Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	272,426	176,869
Level 3 | Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	25	99
Equity securities	12,920	12,541
Arbitrage trading account	26,206	17,308
Assets, fair value disclosure	39,151	29,948
Liabilities:
Securities sold but not yet purchased	14,716	793
Level 3 | Fair Value, Measurements, Recurring | U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	0	0
Level 3 | Fair Value, Measurements, Recurring | State and municipal
Assets:
Total fixed maturity securities available for sale	0	0
Level 3 | Fair Value, Measurements, Recurring | Mortgage-backed securities
Assets:
Total fixed maturity securities available for sale	0	0
Level 3 | Fair Value, Measurements, Recurring | Asset-backed
Assets:
Total fixed maturity securities available for sale	25	99
Level 3 | Fair Value, Measurements, Recurring | Corporate
Assets:
Total fixed maturity securities available for sale	0	0
Level 3 | Fair Value, Measurements, Recurring | Foreign government
Assets:
Total fixed maturity securities available for sale	0	0
Level 3 | Fair Value, Measurements, Recurring | Common stocks
Assets:
Equity securities	8,984	8,596
Level 3 | Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	$ 3,936	$ 3,945