Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Available-for-sale Securities, Debt Securities		$ 4,351,046	$ 4,066,122
Equity securities, AFS		221,951	191,400
Total AFS securities		4,572,997	4,257,522
Short-term investments		125,855	131,972
Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		4,351,046	4,066,122
Equity securities, AFS		221,951	191,400
Total AFS securities		4,572,997	4,257,522
Short-term investments		125,855	131,972
Total assets measured at fair value		4,698,852	4,389,494
Fair Value [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		115,130	124,130
Fair Value [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		19,687	27,831
Fair Value [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		1,338,195	1,246,264
Fair Value [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		1,886,174	1,799,806
Fair Value [Member] | Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		256,923	177,224
Fair Value [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		222,002	179,593
Fair Value [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		512,935	511,274
Fair Value [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member]
Equity securities, AFS		210,197	191,400
Fair Value [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member]
Equity securities, AFS		11,754
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	51,963	53,199
Equity securities, AFS	[1]	219,166	188,500
Total AFS securities	[1]	271,129	241,699
Short-term investments	[1]	125,855	131,972
Total assets measured at fair value	[1]	396,984	373,671
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	51,963	53,199
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member]
Equity securities, AFS	[1]	207,412	188,500
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member]
Equity securities, AFS	[1]	11,754
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	4,299,083	4,012,923
Equity securities, AFS	[1]	0	0
Total AFS securities	[1]	4,299,083	4,012,923
Short-term investments	[1]	0	0
Total assets measured at fair value	[1]	4,299,083	4,012,923
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	63,167	70,931
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	19,687	27,831
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	1,338,195	1,246,264
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	1,886,174	1,799,806
Significant Other Observable Inputs (Level 2) [Member] | Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	256,923	177,224
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	222,002	179,593
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities	[1]	512,935	511,274
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member]
Equity securities, AFS	[1]	0	0
Significant Other Observable Inputs (Level 2) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member]
Equity securities, AFS	[1]	0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		0	0
Equity securities, AFS		2,785	2,900
Total AFS securities		2,785	2,900
Short-term investments		0	0
Total assets measured at fair value		2,785	2,900
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Securities, Debt Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member]
Equity securities, AFS		2,785	$ 2,900
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member]
Equity securities, AFS		$ 0

[1] 1 There were no transfers of securities between Level 1 and Level 2.