Investments Schedule of Available-for-sale and Held-to-Maturity Securities (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		$ 1,148,526	$ 1,182,427
Available-for-sale Securities, Gross Unrealized Gains		2,141	5,072
Available-for-sale Securities, Gross Unrealized Losses		(32,170)	(17,171)
Available-for-sale securities at fair value		1,118,497	1,170,328
Held-to-maturity Securities		70,809	74,576
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		(1,922)	(795)
Held-to-maturity Securities, Fair Value		68,887	73,781
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		35,060	35,132
Available-for-sale Securities, Gross Unrealized Gains		0	0
Available-for-sale Securities, Gross Unrealized Losses		(1,253)	(833)
Available-for-sale securities at fair value		33,807	34,299
US Government-sponsored Enterprises Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		285,992	305,101
Available-for-sale Securities, Gross Unrealized Gains		0	22
Available-for-sale Securities, Gross Unrealized Losses		(5,429)	(2,622)
Available-for-sale securities at fair value		280,563	302,501
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		302,556	299,647
Available-for-sale Securities, Gross Unrealized Gains		1,800	4,559
Available-for-sale Securities, Gross Unrealized Losses		(4,073)	(1,148)
Available-for-sale securities at fair value		300,283	303,058
Mortgage Backed Securities, Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	[1]	508,009	521,753
Available-for-sale Securities, Gross Unrealized Gains	[1]	341	491
Available-for-sale Securities, Gross Unrealized Losses	[1]	(21,415)	(12,568)
Available-for-sale securities at fair value	[1]	486,935	509,676
Held-to-maturity Securities	[1]	70,809	74,576
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	[1]	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	[1]	(1,922)	(795)
Held-to-maturity Securities, Fair Value	[1]	68,887	73,781
Other Aggregated Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		16,909	20,794
Available-for-sale Securities, Gross Unrealized Gains		0	0
Available-for-sale Securities, Gross Unrealized Losses		0	0
Available-for-sale securities at fair value		$ 16,909	$ 20,794

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