Investments Schedule of Available-for-sale and Held-to-Maturity Securities (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		$ 1,135,880	$ 1,084,105
Available-for-sale Securities, Gross Unrealized Gains		17,348	7,634
Available-for-sale Securities, Gross Unrealized Losses		(1,699)	(7,229)
Available-for-sale securities at fair value		1,151,529	1,084,510
Held-to-maturity Securities		111,337	116,352
Held-to-maturity Securities, Unrecognized Holding Gain		1,524	294
Held-to-maturity Securities, Unrecognized Holding Loss		0	(262)
Held-to-maturity Securities, Fair Value		112,861	116,384
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		30,911	21,214
Available-for-sale Securities, Gross Unrealized Gains		481	64
Available-for-sale Securities, Gross Unrealized Losses		(4)	(27)
Available-for-sale securities at fair value		31,388	21,251
US Government-sponsored Enterprises Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		344,945	345,033
Available-for-sale Securities, Gross Unrealized Gains		5,585	874
Available-for-sale Securities, Gross Unrealized Losses		0	(1,345)
Available-for-sale securities at fair value		350,530	344,562
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		264,375	263,661
Available-for-sale Securities, Gross Unrealized Gains		7,334	5,099
Available-for-sale Securities, Gross Unrealized Losses		(95)	(116)
Available-for-sale securities at fair value		271,614	268,644
Mortgage Backed Securities, Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	[1]	477,488	431,446
Available-for-sale Securities, Gross Unrealized Gains	[1]	3,657	1,329
Available-for-sale Securities, Gross Unrealized Losses	[1]	(1,595)	(5,734)
Available-for-sale securities at fair value	[1]	479,550	427,041
Held-to-maturity Securities	[1]	111,337	116,352
Held-to-maturity Securities, Unrecognized Holding Gain	[1]	1,524	294
Held-to-maturity Securities, Unrecognized Holding Loss	[1]	0	(262)
Held-to-maturity Securities, Fair Value	[1]	112,861	116,384
Other Aggregated Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		18,161	22,751
Available-for-sale Securities, Gross Unrealized Gains		291	268
Available-for-sale Securities, Gross Unrealized Losses		(5)	(7)
Available-for-sale securities at fair value		$ 18,447	$ 23,012

[1] All mortgage-backed securities are guaranteed by U.S. government agencies or Government-sponsored entities.