Industry Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	$ 24,895,977				$ 24,299,917				$ 24,895,977	$ 24,299,917
Revenues	1,951,586	$ 1,937,902	$ 1,910,916	$ 1,891,247	1,934,956	$ 2,031,342	$ 1,848,049	$ 1,870,418	7,691,651	7,684,764	$ 7,654,184
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	19,634,329				19,263,193				19,634,329	19,263,193
Revenues									6,456,441	6,229,485	6,148,210
Operating Segments | Insurance | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues									714,200	688,200	733,300
Operating Segments | Reinsurance
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	2,951,115				3,169,731				2,951,115	3,169,731
Revenues									600,815	696,122	777,123
Operating Segments | Reinsurance | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues									228,100	201,300	200,500
Corporate, other and eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	$ 2,310,533				$ 1,866,993				2,310,533	1,866,993
Revenues									$ 479,907	$ 423,299	$ 461,846