12. Equity Compensation Plan Summary of Performance Units Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Equity Compensation Plan [Line Items]
Total Performance Units Outstanding		309,175	349,142	323,989	303,964
Performance Shares [Member]
Equity Compensation Plan [Line Items]
Performance unit fair value at date of grant		$ 38.08	$ 38.34	$ 33.50
Total Performance Units Granted		393,567
Total Performance Units Vested		60,931
Total Performance Units Accelerated		(109,668)
Total Performance Units Forfeited		26,316
Total Performance Units Outstanding		196,652
Total Shares Issued for Vested Performance Units		356,438
Share-based Compensation Arrangement by Share-based Payment Award, Accelerated Vesting, Number			320,676
Share-based Compensation Arrangement by Share-based Payment Award Accelerated Compensation Cost			$ 1.9
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period	57,177
Performance Units Granted 2/23/2012 [Member] | Performance Shares [Member]
Equity Compensation Plan [Line Items]
Performance unit fair value at date of grant		$ 28.26
Total Performance Units Granted		61,033
Total Performance Units Vested		49,936
Total Performance Units Accelerated		0
Total Performance Units Forfeited		11,097
Total Performance Units Outstanding		0
Performance Units Payout Percentage		0.00%
Total Shares Issued for Vested Performance Units		0
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period			0
Performance Units Granted 2/21/2013 [Member] | Performance Shares [Member]
Equity Compensation Plan [Line Items]
Performance unit fair value at date of grant		$ 25.59
Total Performance Units Granted		60,468
Total Performance Units Vested		10,995
Total Performance Units Accelerated		(38,479)
Total Performance Units Forfeited		10,994
Total Performance Units Outstanding		0
Total Shares Issued for Vested Performance Units		148,422
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period		32,985
Performance Units Granted 2/21/2014 [Member] | Performance Shares [Member]
Equity Compensation Plan [Line Items]
Performance unit fair value at date of grant		$ 33.50
Total Performance Units Granted		110,441
Total Performance Units Vested		0
Total Performance Units Accelerated		(70,538)
Total Performance Units Forfeited		1,785
Total Performance Units Outstanding		38,118
Total Shares Issued for Vested Performance Units		207,365
Performance Units Granted 3/16/2015 [Member] | Performance Shares [Member]
Equity Compensation Plan [Line Items]
Performance unit fair value at date of grant		$ 38.34
Total Performance Units Granted		48,674
Total Performance Units Vested		0
Total Performance Units Accelerated		(651)
Total Performance Units Forfeited		2,440
Total Performance Units Outstanding		45,583
Total Shares Issued for Vested Performance Units		651
Performance Units Granted 1/5/2016 [Member] | Performance Shares [Member]
Equity Compensation Plan [Line Items]
Performance unit fair value at date of grant		$ 38.08
Total Performance Units Granted		112,951
Total Performance Units Vested		0
Total Performance Units Accelerated		0
Total Performance Units Forfeited		0
Total Performance Units Outstanding		112,951
Maximum [Member]
Equity Compensation Plan [Line Items]
Performance Units Payout Percentage		(150.00%)
Potential Number Of Shares Issued At Vesting Of Performance Units			300.00%
Share-based Compensation Award, 300% payout [Member] | Performance Units Granted 2/21/2013 [Member] | Performance Shares [Member]
Equity Compensation Plan [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period			115,437
Share-based Compensation Award, 300% payout [Member] | Performance Units Granted 2/21/2014 [Member] | Performance Shares [Member]
Equity Compensation Plan [Line Items]
Performance Units Payout Percentage			(300.00%)
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period			205,240
Share-based Compensation Award, 100% payout [Member] | Performance Units Granted 2/21/2014 [Member] | Performance Shares [Member]
Equity Compensation Plan [Line Items]
Performance Units Payout Percentage			(100.00%)
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period			2,125